SQL Tutorial 2

SQL SELECT INTO Statement

The SQL SELECT INTO statement can be used to create backup copies of tables.

The SQL SELECT INTO Statement

The SELECT INTO statement selects data from one table and inserts it into a different table. 
The SELECT INTO statement is most often used to create backup copies of tables.

SQL SELECT INTO Syntax

We can select all columns into the new table:
SELECT *
INTO new_table_name [IN externaldatabase]
FROM old_tablename
Or we can select only the columns we want into the new table:

SELECT column_name(s)
INTO new_table_name [IN externaldatabase]
FROM old_tablename

SQL SELECT INTO Example

Make a Backup Copy - Now we want to make an exact copy of the data in our "Persons" table.
We use the following SQL statement:
SELECT *
INTO Persons_Backup
FROM Persons
We can also use the IN clause to copy the table into another database:
SELECT *
INTO Persons_Backup IN 'Backup.mdb'
FROM Persons
We can also copy only a few fields into the new table:

SELECT LastName,FirstName
INTO Persons_Backup
FROM Persons

SQL SELECT INTO - With a WHERE Clause

We can also add a WHERE clause.
The following SQL statement creates a "Persons_Backup" table with only the persons who lives in the city "Sandnes":

SELECT LastName,Firstname
INTO Persons_Backup
FROM Persons
WHERE City='Sandnes'

SQL SELECT INTO - Joined Tables

Selecting data from more than one table is also possible.
The following example creates a "Persons_Order_Backup" table contains data from the two tables "Persons" and "Orders":

SELECT Persons.LastName,Orders.OrderNo
INTO Persons_Order_Backup
FROM Persons
INNER JOIN Orders
ON Persons.P_Id=Orders.P_Id

SQL CREATE DATABASE Statement

The CREATE DATABASE Statement

The CREATE DATABASE statement is used to create a database.

SQL CREATE DATABASE Syntax

CREATE DATABASE database_name

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CREATE DATABASE Example

Now we want to create a database called "my_db".
We use the following CREATE DATABASE statement:
CREATE DATABASE my_db
Database tables can be added with the CREATE TABLE statement.

SQL CREATE TABLE Statement

The CREATE TABLE Statement

The CREATE TABLE statement is used to create a table in a database.

SQL CREATE TABLE Syntax

CREATE TABLE table_name
(
column_name1 data_type,
column_name2 data_type,
column_name3 data_type,
....
)
The data type specifies what type of data the column can hold. For a complete reference of all the data types available in MS Access, MySQL, and SQL Server, go to our complete Data Types reference.

CREATE TABLE Example

Now we want to create a table called "Persons" that contains five columns: P_Id, LastName, FirstName, Address, and City.
We use the following CREATE TABLE statement:
CREATE TABLE Persons
(
P_Id int,
LastName varchar(255),
FirstName varchar(255),
Address varchar(255),
City varchar(255)
)
The P_Id column is of type int and will hold a number. The LastName, FirstName, Address, and City columns are of type varchar with a maximum length of 255 characters.
The empty "Persons" table will now look like this:

P_Id	LastName	FirstName	Address	City

The empty table can be filled with data with the INSERT INTO statement.

SQL Constraints

SQL Constraints

Constraints are used to limit the type of data that can go into a table.
Constraints can be specified when a table is created (with the CREATE TABLE statement) or after the table is created (with the ALTER TABLE statement).
We will focus on the following constraints:

• NOT NULL
• UNIQUE
• PRIMARY KEY
• FOREIGN KEY
• CHECK
• DEFAULT 

The next chapters will describe each constraint in detail.

SQL NOT NULL Constraint

By default, a table column can hold NULL values.

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SQL NOT NULL Constraint

The NOT NULL constraint enforces a column to NOT accept NULL values.
The NOT NULL constraint enforces a field to always contain a value. This means that you cannot insert a new record, or update a record without adding a value to this field.
The following SQL enforces the "P_Id" column and the "LastName" column to not accept NULL values:

CREATE TABLE Persons
(
P_Id int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255)
)

SQL UNIQUE Constraint

SQL UNIQUE Constraint

The UNIQUE constraint uniquely identifies each record in a database table.
The UNIQUE and PRIMARY KEY constraints both provide a guarantee for uniqueness for a column or set of columns.
A PRIMARY KEY constraint automatically has a UNIQUE constraint defined on it.
Note that you can have many UNIQUE constraints per table, but only one PRIMARY KEY constraint per table.

SQL UNIQUE Constraint on CREATE TABLE

The following SQL creates a UNIQUE constraint on the "P_Id" column when the "Persons" table is created:
MySQL:
CREATE TABLE Persons
(
P_Id int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255),
UNIQUE (P_Id)
)
SQL Server / Oracle / MS Access:
CREATE TABLE Persons
(
P_Id int NOT NULL UNIQUE,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255)
)
To allow naming of a UNIQUE constraint, and for defining a UNIQUE constraint on multiple columns, use the following SQL syntax:
MySQL / SQL Server / Oracle / MS Access:

CREATE TABLE Persons
(
P_Id int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255),
CONSTRAINT uc_PersonID UNIQUE (P_Id,LastName)
)

SQL UNIQUE Constraint on ALTER TABLE

To create a UNIQUE constraint on the "P_Id" column when the table is already created, use the following SQL:
MySQL / SQL Server / Oracle / MS Access:
ALTER TABLE Persons
ADD UNIQUE (P_Id)
To allow naming of a UNIQUE constraint, and for defining a UNIQUE constraint on multiple columns, use the following SQL syntax:
MySQL / SQL Server / Oracle / MS Access:

ALTER TABLE Persons
ADD CONSTRAINT uc_PersonID UNIQUE (P_Id,LastName)

To DROP a UNIQUE Constraint

To drop a UNIQUE constraint, use the following SQL:
MySQL:
ALTER TABLE Persons
DROP INDEX uc_PersonID
SQL Server / Oracle / MS Access:

ALTER TABLE Persons
DROP CONSTRAINT uc_PersonID

SQL PRIMARY KEY Constraint

SQL PRIMARY KEY Constraint

The PRIMARY KEY constraint uniquely identifies each record in a database table.
Primary keys must contain unique values.
A primary key column cannot contain NULL values.
Each table should have a primary key, and each table can have only ONE primary key.

SQL PRIMARY KEY Constraint on CREATE TABLE

The following SQL creates a PRIMARY KEY on the "P_Id" column when the "Persons" table is created:
MySQL:
CREATE TABLE Persons
(
P_Id int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255),
PRIMARY KEY (P_Id)
)
SQL Server / Oracle / MS Access:
CREATE TABLE Persons
(
P_Id int NOT NULL PRIMARY KEY,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255)
)
To allow naming of a PRIMARY KEY constraint, and for defining a PRIMARY KEY constraint on multiple columns, use the following SQL syntax:
MySQL / SQL Server / Oracle / MS Access:
CREATE TABLE Persons
(
P_Id int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255),
CONSTRAINT pk_PersonID PRIMARY KEY (P_Id,LastName)
)
Note: In the example above there is only ONE PRIMARY KEY (pk_PersonID). However, the value of the pk_PersonID is made up of two columns (P_Id and LastName).

SQL PRIMARY KEY Constraint on ALTER TABLE

To create a PRIMARY KEY constraint on the "P_Id" column when the table is already created, use the following SQL:
MySQL / SQL Server / Oracle / MS Access:
ALTER TABLE Persons
ADD PRIMARY KEY (P_Id)
To allow naming of a PRIMARY KEY constraint, and for defining a PRIMARY KEY constraint on multiple columns, use the following SQL syntax:
MySQL / SQL Server / Oracle / MS Access:
ALTER TABLE Persons
ADD CONSTRAINT pk_PersonID PRIMARY KEY (P_Id,LastName)
Note: If you use the ALTER TABLE statement to add a primary key, the primary key column(s) must already have been declared to not contain NULL values (when the table was first created).

To DROP a PRIMARY KEY Constraint

To drop a PRIMARY KEY constraint, use the following SQL:
MySQL:
ALTER TABLE Persons
DROP PRIMARY KEY
SQL Server / Oracle / MS Access:

ALTER TABLE Persons
DROP CONSTRAINT pk_PersonID

SQL FOREIGN KEY Constraint

A FOREIGN KEY in one table points to a PRIMARY KEY in another table.
Let's illustrate the foreign key with an example. Look at the following two tables:
The "Persons" table:

P_Id	LastName	FirstName	Address	City
1	Hansen	Ola	Timoteivn 10	Sandnes
2	Svendson	Tove	Borgvn 23	Sandnes
3	Pettersen	Kari	Storgt 20	Stavanger

The "Orders" table:

O_Id	OrderNo	P_Id
1	77895	3
2	44678	3
3	22456	2
4	24562	1

Note that the "P_Id" column in the "Orders" table points to the "P_Id" column in the "Persons" table.
The "P_Id" column in the "Persons" table is the PRIMARY KEY in the "Persons" table.
The "P_Id" column in the "Orders" table is a FOREIGN KEY in the "Orders" table.
The FOREIGN KEY constraint is used to prevent actions that would destroy links between tables.
The FOREIGN KEY constraint also prevents that invalid data form being inserted into the foreign key column, because it has to be one of the values contained in the table it points to.

SQL FOREIGN KEY Constraint on CREATE TABLE

The following SQL creates a FOREIGN KEY on the "P_Id" column when the "Orders" table is created:
MySQL:
CREATE TABLE Orders
(
O_Id int NOT NULL,
OrderNo int NOT NULL,
P_Id int,
PRIMARY KEY (O_Id),
FOREIGN KEY (P_Id) REFERENCES Persons(P_Id)
)
SQL Server / Oracle / MS Access:
CREATE TABLE Orders
(
O_Id int NOT NULL PRIMARY KEY,
OrderNo int NOT NULL,
P_Id int FOREIGN KEY REFERENCES Persons(P_Id)
)
To allow naming of a FOREIGN KEY constraint, and for defining a FOREIGN KEY constraint on multiple columns, use the following SQL syntax:
MySQL / SQL Server / Oracle / MS Access:

CREATE TABLE Orders
(
O_Id int NOT NULL,
OrderNo int NOT NULL,
P_Id int,
PRIMARY KEY (O_Id),
CONSTRAINT fk_PerOrders FOREIGN KEY (P_Id)
REFERENCES Persons(P_Id)
)

SQL FOREIGN KEY Constraint on ALTER TABLE

To create a FOREIGN KEY constraint on the "P_Id" column when the "Orders" table is already created, use the following SQL:
MySQL / SQL Server / Oracle / MS Access:
ALTER TABLE Orders
ADD FOREIGN KEY (P_Id)
REFERENCES Persons(P_Id)
To allow naming of a FOREIGN KEY constraint, and for defining a FOREIGN KEY constraint on multiple columns, use the following SQL syntax:
MySQL / SQL Server / Oracle / MS Access:

ALTER TABLE Orders
ADD CONSTRAINT fk_PerOrders
FOREIGN KEY (P_Id)
REFERENCES Persons(P_Id)

To DROP a FOREIGN KEY Constraint

To drop a FOREIGN KEY constraint, use the following SQL:
MySQL:
ALTER TABLE Orders
DROP FOREIGN KEY fk_PerOrders
SQL Server / Oracle / MS Access:

ALTER TABLE Orders
DROP CONSTRAINT fk_PerOrders

SQL CHECK Constraint

The CHECK constraint is used to limit the value range that can be placed in a column.
If you define a CHECK constraint on a single column it allows only certain values for this column.
If you define a CHECK constraint on a table it can limit the values in certain columns based on values in other columns in the row.

SQL CHECK Constraint on CREATE TABLE

The following SQL creates a CHECK constraint on the "P_Id" column when the "Persons" table is created. The CHECK constraint specifies that the column "P_Id" must only include integers greater than 0.
MySQL:
CREATE TABLE Persons
(
P_Id int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255),
CHECK (P_Id>0)
)
SQL Server / Oracle / MS Access:
CREATE TABLE Persons
(
P_Id int NOT NULL CHECK (P_Id>0),
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255)
)
To allow naming of a CHECK constraint, and for defining a CHECK constraint on multiple columns, use the following SQL syntax:
MySQL / SQL Server / Oracle / MS Access:

CREATE TABLE Persons
(
P_Id int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255),
CONSTRAINT chk_Person CHECK (P_Id>0 AND City='Sandnes')
)

SQL CHECK Constraint on ALTER TABLE

To create a CHECK constraint on the "P_Id" column when the table is already created, use the following SQL:
MySQL / SQL Server / Oracle / MS Access:
ALTER TABLE Persons
ADD CHECK (P_Id>0)
To allow naming of a CHECK constraint, and for defining a CHECK constraint on multiple columns, use the following SQL syntax:
MySQL / SQL Server / Oracle / MS Access:

ALTER TABLE Persons
ADD CONSTRAINT chk_Person CHECK (P_Id>0 AND City='Sandnes')

To DROP a CHECK Constraint

To drop a CHECK constraint, use the following SQL:
SQL Server / Oracle / MS Access:
ALTER TABLE Persons
DROP CONSTRAINT chk_Person
MySQL:

ALTER TABLE Persons
DROP CHECK chk_Person

SQL DEFAULT Constraint

The DEFAULT constraint is used to insert a default value into a column.
The default value will be added to all new records, if no other value is specified.

SQL DEFAULT Constraint on CREATE TABLE

The following SQL creates a DEFAULT constraint on the "City" column when the "Persons" table is created:
My SQL / SQL Server / Oracle / MS Access:
CREATE TABLE Persons
(
P_Id int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255) DEFAULT 'Sandnes'
)
The DEFAULT constraint can also be used to insert system values, by using functions like GETDATE():

CREATE TABLE Orders
(
O_Id int NOT NULL,
OrderNo int NOT NULL,
P_Id int,
OrderDate date DEFAULT GETDATE()
)

SQL DEFAULT Constraint on ALTER TABLE

To create a DEFAULT constraint on the "City" column when the table is already created, use the following SQL:
MySQL:
ALTER TABLE Persons
ALTER City SET DEFAULT 'SANDNES'
SQL Server / MS Access:
ALTER TABLE Persons
ALTER COLUMN City SET DEFAULT 'SANDNES'
Oracle:

ALTER TABLE Persons
MODIFY City DEFAULT 'SANDNES'

To DROP a DEFAULT Constraint

To drop a DEFAULT constraint, use the following SQL:
MySQL:
ALTER TABLE Persons
ALTER City DROP DEFAULT
SQL Server / Oracle / MS Access:

ALTER TABLE Persons
ALTER COLUMN City DROP DEFAULT

SQL CREATE INDEX Statement

The CREATE INDEX statement is used to create indexes in tables.
Indexes allow the database application to find data fast; without reading the whole table.

Indexes

An index can be created in a table to find data more quickly and efficiently.
The users cannot see the indexes, they are just used to speed up searches/queries.
Note: Updating a table with indexes takes more time than updating a table without (because the indexes also need an update). So you should only create indexes on columns (and tables) that will be frequently searched against.

SQL CREATE INDEX Syntax

Creates an index on a table. Duplicate values are allowed:
CREATE INDEX index_name
ON table_name (column_name)

SQL CREATE UNIQUE INDEX Syntax

Creates a unique index on a table. Duplicate values are not allowed:
CREATE UNIQUE INDEX index_name
ON table_name (column_name)
Note: The syntax for creating indexes varies amongst different databases. Therefore: Check the syntax for creating indexes in your database.

CREATE INDEX Example

The SQL statement below creates an index named "PIndex" on the "LastName" column in the "Persons" table:
CREATE INDEX PIndex
ON Persons (LastName)
If you want to create an index on a combination of columns, you can list the column names within the parentheses, separated by commas:

CREATE INDEX PIndex
ON Persons (LastName, FirstName)

SQL DROP INDEX, DROP TABLE, and DROP DATABASE

The DROP INDEX Statement

The DROP INDEX statement is used to delete an index in a table.

DROP INDEX Syntax for MS Access:

DROP INDEX index_name ON table_name

DROP INDEX Syntax for MS SQL Server:

DROP INDEX table_name.index_name

DROP INDEX Syntax for DB2/Oracle:

DROP INDEX index_name

DROP INDEX Syntax for MySQL:

ALTER TABLE table_name DROP INDEX index_name

The DROP TABLE Statement

The DROP TABLE statement is used to delete a table.

DROP TABLE table_name

The DROP DATABASE Statement

The DROP DATABASE statement is used to delete a database.

DROP DATABASE database_name

The TRUNCATE TABLE Statement

What if we only want to delete the data inside the table, and not the table itself?
Then, use the TRUNCATE TABLE statement:

TRUNCATE TABLE table_name

SQL ALTER TABLE Statement

The ALTER TABLE Statement

The ALTER TABLE statement is used to add, delete, or modify columns in an existing table.

SQL ALTER TABLE Syntax

To add a column in a table, use the following syntax:
ALTER TABLE table_name
ADD column_name datatype
To delete a column in a table, use the following syntax (notice that some database systems don't allow deleting a column):
ALTER TABLE table_name
DROP COLUMN column_name
To change the data type of a column in a table, use the following syntax:
SQL Server / MS Access:
ALTER TABLE table_name
ALTER COLUMN column_name datatype
My SQL / Oracle:

ALTER TABLE table_name
MODIFY column_name datatype

SQL ALTER TABLE Example

Look at the "Persons" table:

P_Id	LastName	FirstName	Address	City
1	Hansen	Ola	Timoteivn 10	Sandnes
2	Svendson	Tove	Borgvn 23	Sandnes
3	Pettersen	Kari	Storgt 20	Stavanger

Now we want to add a column named "DateOfBirth" in the "Persons" table.
We use the following SQL statement:
ALTER TABLE Persons
ADD DateOfBirth date
Notice that the new column, "DateOfBirth", is of type date and is going to hold a date. The data type specifies what type of data the column can hold. For a complete reference of all the data types available in MS Access, MySQL, and SQL Server, go to our complete Data Types reference.
The "Persons" table will now like this:

P_Id	LastName	FirstName	Address	City	DateOfBirth
1	Hansen	Ola	Timoteivn 10	Sandnes
2	Svendson	Tove	Borgvn 23	Sandnes
3	Pettersen	Kari	Storgt 20	Stavanger

Change Data Type Example

Now we want to change the data type of the column named "DateOfBirth" in the "Persons" table.
We use the following SQL statement:
ALTER TABLE Persons
ALTER COLUMN DateOfBirth year
Notice that the "DateOfBirth" column is now of type year and is going to hold a year in a two-digit or four-digit format.

DROP COLUMN Example

Next, we want to delete the column named "DateOfBirth" in the "Persons" table.
We use the following SQL statement:
ALTER TABLE Persons
DROP COLUMN DateOfBirth
The "Persons" table will now like this:

P_Id	LastName	FirstName	Address	City
1	Hansen	Ola	Timoteivn 10	Sandnes
2	Svendson	Tove	Borgvn 23	Sandnes
3	Pettersen	Kari	Storgt 20	Stavanger

SQL AUTO INCREMENT Field

Auto-increment allows a unique number to be generated when a new record is inserted into a table.

AUTO INCREMENT a Field

Very often we would like the value of the primary key field to be created automatically every time a new record is inserted.
We would like to create an auto-increment field in a table.

Syntax for MySQL

The following SQL statement defines the "P_Id" column to be an auto-increment primary key field in the "Persons" table:
CREATE TABLE Persons
(
P_Id int NOT NULL AUTO_INCREMENT,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255),
PRIMARY KEY (P_Id)
)
MySQL uses the AUTO_INCREMENT keyword to perform an auto-increment feature.
By default, the starting value for AUTO_INCREMENT is 1, and it will increment by 1 for each new record.
To let the AUTO_INCREMENT sequence start with another value, use the following SQL statement:
ALTER TABLE Persons AUTO_INCREMENT=100
To insert a new record into the "Persons" table, we will not have to specify a value for the "P_Id" column (a unique value will be added automatically):
INSERT INTO Persons (FirstName,LastName)
VALUES ('Lars','Monsen')
The SQL statement above would insert a new record into the "Persons" table. The "P_Id" column would be assigned a unique value. The "FirstName" column would be set to "Lars" and the "LastName" column would be set to "Monsen".

Syntax for SQL Server

The following SQL statement defines the "P_Id" column to be an auto-increment primary key field in the "Persons" table:
CREATE TABLE Persons
(
P_Id int PRIMARY KEY IDENTITY,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255)
)
The MS SQL Server uses the IDENTITY keyword to perform an auto-increment feature. 
By default, the starting value for IDENTITY is 1, and it will increment by 1 for each new record.
To specify that the "P_Id" column should start at value 10 and increment by 5, change the identity to IDENTITY(10,5).
To insert a new record into the "Persons" table, we will not have to specify a value for the "P_Id" column (a unique value will be added automatically):
INSERT INTO Persons (FirstName,LastName)
VALUES ('Lars','Monsen')
The SQL statement above would insert a new record into the "Persons" table. The "P_Id" column would be assigned a unique value. The "FirstName" column would be set to "Lars" and the "LastName" column would be set to "Monsen".

Syntax for Access

The following SQL statement defines the "P_Id" column to be an auto-increment primary key field in the "Persons" table:
CREATE TABLE Persons
(
P_Id PRIMARY KEY AUTOINCREMENT,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255)
)
The MS Access uses the AUTOINCREMENT keyword to perform an auto-increment feature. 
By default, the starting value for AUTOINCREMENT is 1, and it will increment by 1 for each new record.
To specify that the "P_Id" column should start at value 10 and increment by 5, change the autoincrement to AUTOINCREMENT(10,5).
To insert a new record into the "Persons" table, we will not have to specify a value for the "P_Id" column (a unique value will be added automatically):
INSERT INTO Persons (FirstName,LastName)
VALUES ('Lars','Monsen')
The SQL statement above would insert a new record into the "Persons" table. The "P_Id" column would be assigned a unique value. The "FirstName" column would be set to "Lars" and the "LastName" column would be set to "Monsen".

Syntax for Oracle

In Oracle the code is a little bit more tricky.
You will have to create an auto-increment field with the sequence object (this object generates a number sequence).
Use the following CREATE SEQUENCE syntax:
CREATE SEQUENCE seq_person
MINVALUE 1
START WITH 1
INCREMENT BY 1
CACHE 10
The code above creates a sequence object called seq_person, that starts with 1 and will increment by 1. It will also cache up to 10 values for performance. The cache option specifies how many sequence values will be stored in memory for faster access.
To insert a new record into the "Persons" table, we will have to use the nextval function (this function retrieves the next value from seq_person sequence):
INSERT INTO Persons (P_Id,FirstName,LastName)
VALUES (seq_person.nextval,'Lars','Monsen')
The SQL statement above would insert a new record into the "Persons" table. The "P_Id" column would be assigned the next number from the seq_person sequence. The "FirstName" column would be set to "Lars" and the "LastName" column would be set to "Monsen".

SQL Views

A view is a virtual table.
This chapter shows how to create, update, and delete a view. 

SQL CREATE VIEW Statement

In SQL, a view is a virtual table based on the result-set of an SQL statement.
A view contains rows and columns, just like a real table. The fields in a view are fields from one or more real tables in the database.
You can add SQL functions, WHERE, and JOIN statements to a view and present the data as if the data were coming from one single table.

SQL CREATE VIEW Syntax

CREATE VIEW view_name AS
SELECT column_name(s)
FROM table_name
WHERE condition
Note: A view always shows up-to-date data! The database engine recreates the data, using the view's SQL statement, every time a user queries a view.

SQL CREATE VIEW Examples

If you have the Northwind database you can see that it has several views installed by default.
The view "Current Product List" lists all active products (products that are not discontinued) from the "Products" table. The view is created with the following SQL:
CREATE VIEW [Current Product List] AS
SELECT ProductID,ProductName
FROM Products
WHERE Discontinued=No
We can query the view above as follows:
SELECT * FROM [Current Product List]
Another view in the Northwind sample database selects every product in the "Products" table with a unit price higher than the average unit price:
CREATE VIEW [Products Above Average Price] AS
SELECT ProductName,UnitPrice
FROM Products
WHERE UnitPrice>(SELECT AVG(UnitPrice) FROM Products)
We can query the view above as follows:
SELECT * FROM [Products Above Average Price]
Another view in the Northwind database calculates the total sale for each category in 1997. Note that this view selects its data from another view called "Product Sales for 1997":
CREATE VIEW [Category Sales For 1997] AS
SELECT DISTINCT CategoryName,Sum(ProductSales) AS CategorySales
FROM [Product Sales for 1997]
GROUP BY CategoryName
We can query the view above as follows:
SELECT * FROM [Category Sales For 1997]
We can also add a condition to the query. Now we want to see the total sale only for the category "Beverages":

SELECT * FROM [Category Sales For 1997]
WHERE CategoryName='Beverages'

SQL Updating a View

You can update a view by using the following syntax:

SQL CREATE OR REPLACE VIEW Syntax

CREATE OR REPLACE VIEW view_name AS
SELECT column_name(s)
FROM table_name
WHERE condition
Now we want to add the "Category" column to the "Current Product List" view. We will update the view with the following SQL:

CREATE VIEW [Current Product List] AS
SELECT ProductID,ProductName,Category
FROM Products
WHERE Discontinued=No

SQL Dropping a View

You can delete a view with the DROP VIEW command.

SQL DROP VIEW Syntax

DROP VIEW view_name

SQL Date Functions

SQL Dates

 The most difficult part when working with dates is to be sure that the format of the date you are trying to insert, matches the format of the date column in the database.
As long as your data contains only the date portion, your queries will work as expected. However, if a time portion is involved, it gets complicated.
Before talking about the complications of querying for dates, we will look at the most important built-in functions for working with dates.

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MySQL Date Functions

The following table lists the most important built-in date functions in MySQL:

Function	Description
NOW()	Returns the current date and time
CURDATE()	Returns the current date
CURTIME()	Returns the current time
DATE()	Extracts the date part of a date or date/time expression
EXTRACT()	Returns a single part of a date/time
DATE_ADD()	Adds a specified time interval to a date
DATE_SUB()	Subtracts a specified time interval from a date
DATEDIFF()	Returns the number of days between two dates
DATE_FORMAT()	Displays date/time data in different formats

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SQL Server Date Functions

The following table lists the most important built-in date functions in SQL Server:

Function	Description
GETDATE()	Returns the current date and time
DATEPART()	Returns a single part of a date/time
DATEADD()	Adds or subtracts a specified time interval from a date
DATEDIFF()	Returns the time between two dates
CONVERT()	Displays date/time data in different formats

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SQL Date Data Types

MySQL comes with the following data types for storing a date or a date/time value in the database:

• DATE - format YYYY-MM-DD
• DATETIME - format: YYYY-MM-DD HH:MM:SS
• TIMESTAMP - format: YYYY-MM-DD HH:MM:SS
• YEAR - format YYYY or YY 

SQL Server comes with the following data types for storing a date or a date/time value in the database:

• DATE - format YYYY-MM-DD
• DATETIME - format: YYYY-MM-DD HH:MM:SS
• SMALLDATETIME - format: YYYY-MM-DD HH:MM:SS
• TIMESTAMP - format: a unique number 

Note: The date types are chosen for a column when you create a new table in your database!
For an overview of all data types available, go to our complete Data Types reference.

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SQL Working with Dates

 You can compare two dates easily if there is no time component involved!
Assume we have the following "Orders" table:

OrderId	ProductName	OrderDate
1	Geitost	2008-11-11
2	Camembert Pierrot	2008-11-09
3	Mozzarella di Giovanni	2008-11-11
4	Mascarpone Fabioli	2008-10-29

Now we want to select the records with an OrderDate of "2008-11-11" from the table above.
We use the following SELECT statement:
SELECT * FROM Orders WHERE OrderDate='2008-11-11'
The result-set will look like this:

OrderId	ProductName	OrderDate
1	Geitost	2008-11-11
3	Mozzarella di Giovanni	2008-11-11

Now, assume that the "Orders" table looks like this (notice the time component in the "OrderDate" column):

OrderId	ProductName	OrderDate
1	Geitost	2008-11-11 13:23:44
2	Camembert Pierrot	2008-11-09 15:45:21
3	Mozzarella di Giovanni	2008-11-11 11:12:01
4	Mascarpone Fabioli	2008-10-29 14:56:59

If we use the same SELECT statement as above:
SELECT * FROM Orders WHERE OrderDate='2008-11-11'
we will get no result! This is because the query is looking only for dates with no time portion.
Tip: If you want to keep your queries simple and easy to maintain, do not allow time components in your dates!

SQL NULL Values

NULL values represent missing unknown data.
By default, a table column can hold NULL values.
This chapter will explain the IS NULL and IS NOT NULL operators.

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SQL NULL Values

If a column in a table is optional, we can insert a new record or update an existing record without adding a value to this column. This means that the field will be saved with a NULL value.
NULL values are treated differently from other values.
NULL is used as a placeholder for unknown or inapplicable values.
 Note: It is not possible to compare NULL and 0; they are not equivalent.

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SQL Working with NULL Values

Look at the following "Persons" table:

P_Id	LastName	FirstName	Address	City
1	Hansen	Ola		Sandnes
2	Svendson	Tove	Borgvn 23	Sandnes
3	Pettersen	Kari		Stavanger

Suppose that the "Address" column in the "Persons" table is optional. This means that if we insert a record with no value for the "Address" column, the "Address" column will be saved with a NULL value.
How can we test for NULL values?
It is not possible to test for NULL values with comparison operators, such as =, <, or <>.
We will have to use the IS NULL and IS NOT NULL operators instead.

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SQL IS NULL

How do we select only the records with NULL values in the "Address" column?
We will have to use the IS NULL operator:
SELECT LastName,FirstName,Address FROM Persons
WHERE Address IS NULL
The result-set will look like this:

LastName	FirstName	Address
Hansen	Ola
Pettersen	Kari

 Tip: Always use IS NULL to look for NULL values.

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SQL IS NOT NULL

How do we select only the records with no NULL values in the "Address" column?
We will have to use the IS NOT NULL operator:
SELECT LastName,FirstName,Address FROM Persons
WHERE Address IS NOT NULL
The result-set will look like this:

LastName	FirstName	Address
Svendson	Tove	Borgvn 23

In the next chapter we will look at the ISNULL(), NVL(), IFNULL() and COALESCE() functions.

SQL NULL Functions

SQL ISNULL(), NVL(), IFNULL() and COALESCE() Functions

Look at the following "Products" table:

P_Id	ProductName	UnitPrice	UnitsInStock	UnitsOnOrder
1	Jarlsberg	10.45	16	15
2	Mascarpone	32.56	23
3	Gorgonzola	15.67	9	20

Suppose that the "UnitsOnOrder" column is optional, and may contain NULL values.
We have the following SELECT statement:
SELECT ProductName,UnitPrice*(UnitsInStock+UnitsOnOrder)
FROM Products
In the example above, if any of the "UnitsOnOrder" values are NULL, the result is NULL.
Microsoft's ISNULL() function is used to specify how we want to treat NULL values.
The NVL(), IFNULL(), and COALESCE() functions can also be used to achieve the same result.
In this case we want NULL values to be zero.
Below, if "UnitsOnOrder" is NULL it will not harm the calculation, because ISNULL() returns a zero if the value is NULL:
SQL Server / MS Access
SELECT ProductName,UnitPrice*(UnitsInStock+ISNULL(UnitsOnOrder,0))
FROM Products
Oracle
Oracle does not have an ISNULL() function. However, we can use the NVL() function to achieve the same result:
SELECT ProductName,UnitPrice*(UnitsInStock+NVL(UnitsOnOrder,0))
FROM Products
MySQL
MySQL does have an ISNULL() function. However, it works a little bit different from Microsoft's ISNULL() function.
In MySQL we can use the IFNULL() function, like this:
SELECT ProductName,UnitPrice*(UnitsInStock+IFNULL(UnitsOnOrder,0))
FROM Products
or we can use the COALESCE() function, like this:

SELECT ProductName,UnitPrice*(UnitsInStock+COALESCE(UnitsOnOrder,0))
FROM Products