sql语句基础学习

sql select语句详解

SELECT [ ALL | DISTINCT [ ON ( expression [, ...] ) ] ]

* | expression [ AS output_name ] [, ...]

[ FROM from_item [, ...] ]

[ WHERE condition ]

[ GROUP BY expression [, ...] ]

[ HAVING condition [, ...] ]

[ { UNION | INTERSECT | EXCEPT } [ ALL ] select ]

[ ORDER BY expression [ ASC | DESC | USING operator ] [, ...] ]

[ FOR UPDATE [ OF tablename [, ...] ] ]

[ LIMIT { count | ALL } ]

[ OFFSET start ]

这里 from_item 可以是：

[ ONLY ] table_name [ * ]

[ [ AS ] alias [ ( column_alias_list ) ] ]

|

( select )

[ AS ] alias [ ( column_alias_list ) ]

|

from_item [ NATURAL ] join_type from_item

[ ON join_condition | USING ( join_column_list ) ]

输入

expression

表的列/字段名或一个表达式．

output_name

使用 AS 子句为一个列/字段或一个表达式声明另一个名称． 这个名称主要用于标记输出列用于显示。 它可以在 ORDER BY 和 GROUP BY 子句里代表列/字段的值． 但是 output_name 不能用于 WHERE 或 HAVING 子句；用表达式代替．

from_item

一个表引用，子查询，或者 JOIN 子句．详见下文．

condition

一个布尔表达式，给出真或假的结果． 参见下面描述的 WHERE 和 HAVING 子句．