SQL-W3School: SQL WHERE statement

ylbtech-SQL-W3School: SQL WHERE statement

 

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WHERE clause for selection standards.

WHERE clause

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

grammar

SELECT column names FROM table name WHERE column operator value

The following operators can be used in the WHERE clause:

Operators	description
=	equal
<>	not equal to
>	more than the
<	Less than
>=	greater or equal to
<=	Less than or equal
BETWEEN	Within a certain range
LIKE	Search a pattern

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

Using the WHERE clause

If you only want to select people living in the city "Beijing" in, we need to add a WHERE clause to the SELECT statement:

SELECT * FROM Persons WHERE City='Beijing'

"Persons" 表

LastName	FirstName	Address	City	Year
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

result:

LastName	FirstName	Address	City	Year
Carter	Thomas	Changan Street	Beijing	1980
Gates	Bill	Xuanwumen 10	Beijing	1985

Use of quotes

Please note that we value in case the conditions of use is surrounded by single quotes.

SQL uses single quotes surround text values (most database systems also accept double quotes). If the value, do not use quotation marks .

Text Value:

this is correct:
SELECT * FROM Persons WHERE FirstName='Bush'

This is wrong:
SELECT * FROM Persons WHERE FirstName=Bush

Value:

this is correct:
SELECT * FROM Persons WHERE Year>1965

This is wrong:
SELECT * FROM Persons WHERE Year>'1965'

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https://www.w3school.com.cn/sql/sql_where.asp

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