Tuesday 3 April 2012

SQL Quick Reference

Posted by Nag , at 12:37

The INSERT INTO Statement

The INSERT INTO statement is used to insert new rows into a table.

Syntax

INSERT INTO table_name
VALUES (value1, value2,....)
You can also specify the columns for which you want to insert data:
INSERT INTO table_name (column1, column2,...)
VALUES (value1, value2,....)


Insert a New Row

This "Persons" table:
LastName
FirstName
Address
City
Pettersen
Kari
Storgt 20
Stavanger
And this SQL statement:
INSERT INTO Persons 
VALUES ('Hetland', 'Camilla', 'Hagabakka 24', 'Sandnes')
Will give this result:
LastName
FirstName
Address
City
Pettersen
Kari
Storgt 20
Stavanger
Hetland
Camilla
Hagabakka 24
Sandnes


Insert Data in Specified Columns

This "Persons" table:
LastName
FirstName
Address
City
Pettersen
Kari
Storgt 20
Stavanger
Hetland
Camilla
Hagabakka 24
Sandnes
And This SQL statement:
INSERT INTO Persons (LastName, Address)
VALUES ('Rasmussen', 'Storgt 67')
Will give this result:
LastName
FirstName
Address
City
Pettersen
Kari
Storgt 20
Stavanger
Hetland
Camilla
Hagabakka 24
Sandnes
Rasmussen

Storgt 67



The Update Statement

The UPDATE statement is used to modify the data in a table.

Syntax

UPDATE table_name
SET column_name = new_value
WHERE column_name = some_value


Person:
LastName
FirstName
Address
City
Nilsen
Fred
Kirkegt 56
Stavanger
Rasmussen

Storgt 67



Update one Column in a Row

We want to add a first name to the person with a last name of "Rasmussen":
UPDATE Person SET FirstName = 'Nina'
WHERE LastName = 'Rasmussen'
Result:
LastName
FirstName
Address
City
Nilsen
Fred
Kirkegt 56
Stavanger
Rasmussen
Nina
Storgt 67



Update several Columns in a Row

We want to change the address and add the name of the city:
UPDATE Person
SET Address = 'Stien 12', City = 'Stavanger'
WHERE LastName = 'Rasmussen'
Result:
LastName
FirstName
Address
City
Nilsen
Fred
Kirkegt 56
Stavanger
Rasmussen
Nina
Stien 12
Stavanger



The DELETE Statement

The DELETE statement is used to delete rows in a table.

Syntax

DELETE FROM table_name
WHERE column_name = some_value


Person:
LastName
FirstName
Address
City
Nilsen
Fred
Kirkegt 56
Stavanger
Rasmussen
Nina
Stien 12
Stavanger


Delete a Row

"Nina Rasmussen" is going to be deleted:
DELETE FROM Person WHERE LastName = 'Rasmussen'
Result
LastName
FirstName
Address
City
Nilsen
Fred
Kirkegt 56
Stavanger


Delete All Rows

It is possible to delete all rows in a table without deleting the table. This means that the table structure, attributes, and indexes will be intact:
DELETE FROM table_name
or
DELETE * FROM table_name




The SQL SELECT Statement

The SELECT statement is used to select data from a table. The tabular result is stored in a result table (called the result-set).

Syntax

SELECT column_name(s)
FROM table_name
NoteNote: SQL statements are not case sensitive. SELECT is the same as select.

SQL SELECT Example

To select the content of columns named "LastName" and "FirstName", from the database table called "Persons", use a SELECT statement like this:
SELECT LastName,FirstName FROM Persons
The database table "Persons":
LastName
FirstName
Address
City
Hansen
Ola
Timoteivn 10
Sandnes
Svendson
Tove
Borgvn 23
Sandnes
Pettersen
Kari
Storgt 20
Stavanger
The result
LastName
FirstName
Hansen
Ola
Svendson
Tove
Pettersen
Kari


Select All Columns

To select all columns from the "Persons" table, use a * symbol instead of column names, like this: 
SELECT * FROM Persons
Result
LastName
FirstName
Address
City
Hansen
Ola
Timoteivn 10
Sandnes
Svendson
Tove
Borgvn 23
Sandnes
Pettersen
Kari
Storgt 20
Stavanger


The Result Set

The result from a SQL query is stored in a result-set. Most database software systems allow navigation of the result set with programming functions, like: Move-To-First-Record, Get-Record-Content, Move-To-Next-Record, etc.
Programming functions like these are not a part of this tutorial. To learn about accessing data with function calls, please visit our ADO tutorial.

Semicolon after SQL Statements?

Semicolon is the standard way to separate each SQL statement in database systems that allow more than one SQL statement to be executed in the same call to the server.
Some SQL tutorials end each SQL statement with a semicolon. Is this necessary? We are using MS Access and SQL Server 2000 and we do not have to put a semicolon after each SQL statement, but some database programs force you to use it.

The SELECT DISTINCT Statement

The DISTINCT keyword is used to return only distinct (different) values.
The SELECT statement returns information from table columns. But what if we only want to select distinct elements?
With SQL, all we need to do is to add a DISTINCT keyword to the SELECT statement:

Syntax

SELECT DISTINCT column_name(s)
FROM table_name


Using the DISTINCT keyword

To select ALL values from the column named "Company" we use a SELECT statement like this:
SELECT Company FROM Orders
"Orders" table
Company
OrderNumber
Sega
3412
W3Schools
2312
Trio
4678
W3Schools
6798
Result
Company
Sega
W3Schools
Trio
W3Schools
Note that "W3Schools" is listed twice in the result-set.
To select only DIFFERENT values from the column named "Company" we use a SELECT DISTINCT statement like this:
SELECT DISTINCT Company FROM Orders
Result:
Company
Sega
W3Schools
Trio





The WHERE Clause 

To conditionally select data from a table, a WHERE clause can be added to the SELECT statement.

Syntax

SELECT column FROM table
WHERE column operator value
With the WHERE clause, the following operators can be used:
Operator
Description
=
Equal
<> 
Not equal
> 
Greater than
< 
Less than
>=
Greater than or equal
<=
Less than or equal
BETWEEN
Between an inclusive range
LIKE
 Search for a pattern
IN
If you know the exact value you want to return for at least one of the columns
Note: In some versions of SQL the <> operator may be written as !=

Using the WHERE Clause

To select only the persons living in the city "Sandnes", we add a WHERE clause to the SELECT statement: 
SELECT * FROM Persons
WHERE City='Sandnes'
"Persons" table
LastName
FirstName
Address
City
Year
Hansen
Ola
Timoteivn 10
Sandnes
1951
Svendson
Tove
Borgvn 23
Sandnes
1978
Svendson
Stale
Kaivn 18
Sandnes
1980
Pettersen
Kari
Storgt 20
Stavanger
1960
Result
LastName
FirstName
Address
City
Year
Hansen
Ola
Timoteivn 10
Sandnes
1951
Svendson
Tove
Borgvn 23
Sandnes
1978
Svendson
Stale
Kaivn 18
Sandnes
1980


Using Quotes

Note that we have used single quotes around the conditional values in the examples.
SQL uses single quotes around text values (most database systems will also accept double quotes). Numeric values should not be enclosed in quotes.
For text values:
This is correct:
SELECT * FROM Persons WHERE FirstName='Tove'
This is wrong:
SELECT * FROM Persons WHERE FirstName=Tove
For numeric values:
This is correct:
SELECT * FROM Persons WHERE Year>1965
This is wrong:
SELECT * FROM Persons WHERE Year>'1965'


The LIKE Condition

The LIKE condition is used to specify a search for a pattern in a column.

Syntax

SELECT column FROM table
WHERE column LIKE pattern
A "%" sign can be used to define wildcards (missing letters in the pattern) both before and after the pattern.

Using LIKE

The following SQL statement will return persons with first names that start with an 'O':
SELECT * FROM Persons
WHERE FirstName LIKE 'O%'
The following SQL statement will return persons with first names that end with an 'a':
SELECT * FROM Persons
WHERE FirstName LIKE '%a'
The following SQL statement will return persons with first names that contain the pattern 'la':
SELECT * FROM Persons
WHERE FirstName LIKE '%la%'

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