语法
select 查询列表 from 表名 where 查询条件;
查询顺序
先查对应的表,根据查询条件筛选表中记录,显示符合条件的对应查询列表
分类
- 按条件表达式查询
条件运算符 :>,<,=,!=,<>,<=,>=
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大于: select * from `employees` where `salary`>3000 不等于: select `email`,`salary` from `employees` where `salary`!=10000; 或 select `email`,`salary` from `employees` where `salary`<>10000;
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按逻辑表达式查询
逻辑运算符:用于连接条件运算符 and or not(对应于java中的&&,||,!)
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select `email` from `employees`where `salary`>=3000 and `salary`<=5000 select * from `employees` where not (`department_id`>=90 and `department_id`<=110) or `salary`>15000
- 模糊查询
查询关键字:like,between...and...,in,IS NULL(为空),IS NOT NULL(不为空)
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select * from `employees` where `last_name` like '%li%'; select * from `employees` where `last_name` like '__n_l%'; select * from `employees` where `last_name` like '_a_%' ESCAPE 'a'; like关键字通常与通配符一起使用 通配符: %:任意多个字符,包括零个 _: 任意单个字符 注:通过ESCAPE可将任意一个字符指定为转义字符。
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select `email` from `employees`where `salary` between 3000 and 5000 between(x)and(y):指处在x~y之间的值(包含临界值)
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select * from `employees` where `salary` in(3000,5000,10000); in(x,y,z,...):满足查询条件x,y,z...中的一个即可 条件值得类型必须一致或兼容
排序查询
- 语法
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select 查询列表 from 表名 [where 查询条件] order by 排序列表 【asc|desc】; 从高到低:select * from `employees` order by `salary` desc; 从低到高:select * from `employees` order by `salary` asc;
- 查询示例
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按函数排序:select * from `employees` ORDER BY length(`last_name`) ASC; 先降序再升序:SELECT * FROM `employees` ORDER BY `salary` DESC , LENGTH(`last_name`) ASC