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复制
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 ] ]复制
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复制
返回结果:
LastName | FirstName |
Hansen | Ola |
Svendson | Tove |
Pettersen | Kari |
选出所有字段的数据
SELECT * FROM Persons复制
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复制
=,<>,>,<,>=,<=,BETWEEN,LIKE下面的操作符能被使用在WHERE中：
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'复制
"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'复制
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'复制
返回结果:
LastName | FirstName | Address | City |
Svendson | Tove | Borgvn 23 | Sandnes |
Svendson | Stephen | Kaivn 18 | Sandnes |
你也能结合AND和OR (使用括号形成复杂的表达式),如:
SELECT * FROM Persons WHERE (FirstName='Tove' OR FirstName='Stephen') AND LastName='Svendson'复制
返回结果:
LastName | FirstName | Address | City |
Svendson | Tove | Borgvn 23 | Sandnes |
Svendson | Stephen | Kaivn 18 | Sandnes |
