Select
用途：
从指定表中取出指定的列的数据
语法：
SELECT column_name(s) FROM table_name
|
解释：
从数据库中选取资料列，并允许从一或多个资料表中，选取一或多个资料列或资料行。SELECT 陈述式的完整语法相当复杂，但主要子句可摘要为：
SELECT select_list
[ INTO new_table ]
FROM table_source
[ WHERE search_condition ]
[ GROUP BY group_by_expression ]
[ HAVING search_condition ]
[ ORDER BY order_expression [ ASC | DESC ] ]
例：
“Persons” 表中的数据有
|
LastName
|
FirstName
|
Address
|
City
|
|
Hansen
|
Ola
|
Timoteivn 10
|
Sandnes
|
|
Svendson
|
Tove
|
Borgvn 23
|
Sandnes
|
|
Pettersen
|
Kari
|
Storgt 20
|
Stavanger
|
选出字段名” LastName”、” FirstName” 的数据
SELECT LastName,FirstName FROM Persons
|
返回结果:
|
LastName
|
FirstName
|
|
Hansen
|
Ola
|
|
Svendson
|
Tove
|
|
Pettersen
|
Kari
|
选出所有字段的数据
返回结果:
|
LastName
|
FirstName
|
Address
|
City
|
|
Hansen
|
Ola
|
Timoteivn 10
|
Sandnes
|
|
Svendson
|
Tove
|
Borgvn 23
|
Sandnes
|
|
Pettersen
|
Kari
|
Storgt 20
|
Stavanger
|
Where
用途：
被用来规定一种选择查询的标准
语法：
SELECT column FROM table WHERE column condition value
|
下面的操作符能被使用在WHERE中：
=,<>,>,<,>=,<=,BETWEEN,LIKE
注意： 在某些SQL的版本中不等号< >能被写作为!=
解释：
SELECT语句返回WHERE子句中条件为true的数据
例：
从” Persons”表中选出生活在” Sandnes” 的人
SELECT * FROM Persons WHERE City='Sandnes'
|
"Persons" 表中的数据有:
|
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
|
返回结果:
|
LastName
|
FirstName
|
Address
|
City
|
Year
|
|
Hansen
|
Ola
|
Timoteivn 10
|
Sandnes
|
1951
|
|
Svendson
|
Tove
|
Borgvn 23
|
Sandnes
|
1978
|
|
Svendson
|
Stale
|
Kaivn 18
|
Sandnes
|
1980
|
And & Or
用途：
在WHERE子句中AND和OR被用来连接两个或者更多的条件
解释：
AND在结合两个布尔表达式时，只有在两个表达式都为 TRUE 时才传回 TRUE
OR在结合两个布尔表达式时，只要其中一个条件为 TRUE 时，OR便传回 TRUE
例：
"Persons" 表中的原始数据:
|
LastName
|
FirstName
|
Address
|
City
|
|
Hansen
|
Ola
|
Timoteivn 10
|
Sandnes
|
|
Svendson
|
Tove
|
Borgvn 23
|
Sandnes
|
|
Svendson
|
Stephen
|
Kaivn 18
|
Sandnes
|
用AND运算子来查找"Persons" 表中FirstName为”Tove”而且LastName为” Svendson”的数据
SELECT * FROM Persons
WHERE FirstName='Tove'
AND LastName='Svendson'
|
返回结果:
|
LastName
|
FirstName
|
Address
|
City
|
|
Svendson
|
Tove
|
Borgvn 23
|
Sandnes
|
用OR运算子来查找"Persons" 表中FirstName为”Tove”或者LastName为” Svendson”的数据
SELECT * FROM Persons
WHERE firstname='Tove'
OR lastname='Svendson'
|
返回结果:
|
LastName
|
FirstName
|
Address
|
City
|
|
Svendson
|
Tove
|
Borgvn 23
|
Sandnes
|
|
Svendson
|
Stephen
|
|