Some basic SQL operations (SELECT, DISTINCT, WHERE, quotes)

The data table used in the following example

City Year

LastName	FirstName	Address
Adams	John	Oxford Street	London	1970
Bush	George	Fifth Avenue	New York	1975
Carter	Thomas	Changan Street	Beijing	1980
Gates	Bill	Xuanwumen 10	Beijing	1985

SELECT

Select statements are used to select data from a table, and the results are stored in a result table, called a result set. Common syntax is as follows

1. Select all the corresponding columns 1 and 2 from the data table

SELECT column 1 name, column 2 name from table name

2. Select the data from the data table for all columns , and the asterisk (*) is a shortcut for selecting all columns.

SELECT * FROM table name

3. In the table, duplicate values may be included. That's not a problem, but sometimes you might want to just list different values (distinct). Keyword DISTINCT is used to return only different values, and repeated values will display only one in the result set.

SELECT DISTINCT column name from table name

Example

SELECT City DISTINCT from Persons

　　Navigating in the result set (Result-set)

The results obtained by the SQL query program are stored in a result set. Most database software systems allow the use of programming functions to navigate the result set, such as: Move-to-first-record, Get-record-content, Move-to-next-record, and so on.

Similar to these programming functions are not listed in this tutorial. To learn about accessing data through function calls, please visit our ADO tutorials and PHP tutorials.

WHERE

The WHERE clause is used to specify the criteria for selection. If you need to conditionally select data from a table, you can add a WHERE clause to the SELECT statement with the following syntax

SELECT column name from table name WHERE column operator value

The following operators can be used in the WHERE clause

operator	Description
=	Equals
<>	Not equal to
>	Greater than
<	Less than
>=	Greater than or equal
<=	Less than or equal
Between	Within a range
Like	Search for a pattern

Note: In some versions of SQL, the operator <> can be written as! =.

For example

SELECT * from Persons WHERE city = ' Beijing '

　　

Use of quotation marks

Note that the above condition values in the example use single quotes around the

SQL uses single quotes around text values (most database systems also accept double quotes). If it is numeric , do not use quotation marks

Text value

This is the correct select * from Persons where firstname= ' Bush ' This is the wrong select * from Persons where Firstname=bush

Numerical

This is the correct select * from Persons where year>1965 this is the wrong select * from Persons where year> ' 1965 '

　　

Some basic SQL operations (SELECT, DISTINCT, WHERE, quotes)