数据库(2) SQL查询--简单查询

SQL查询一般包括： 简单查询，连接查询，聚合查询，嵌套子查询。

简单查询

select * from tableName //查询所有的列select DISTINCT columnName from tableName //查询指定的列并消除重复的元组select columnName 别名 from tableName //给查询的列取别名select columnName AS 别名 from tableName

下面是where 子句中常用的查询条件：

1.比较运算：>，<，=，>=，<=，<>（!=）；

select * from tableName where num >=20

2.范围查询：BETWEEN … AND …

select * from tableName where score BETWEEN 60 AND 90

3.集合查询：IN

select * from tableName where classNo IN ('001','002')select * from tableName where classNo NOT IN ('001','002')

4.空值查询：IS null

select * from tableName where class IS null

5.字符匹配查询：LIKE；

select * from tableName where name LIKE '%小%'select * from tableName where name LIKE '陈_ _'select * from tableName where className LIKE '%08/_%' ESCAPE '/'

6.逻辑查询：AND，OR，NOT。

select * from tableName where class IS null AND name LIKE '%小%'