关于mysql:MySQL-select查询教程

查问 Syntax:

SELECT
[* | DISTINCT | col_name | col_name as new_name]
FROM table
WHERE condition
GROUP BY {col_name} HAVING where_condition
ORDER BY {col_name} [ASC | DESC]
LIMIT {[offset,] row_count | row_count OFFSET offset}

。select * from ..

select from emp; 会查问所有的记录行；生产中杜绝应用（select from emp; 会做全表扫描）

select * from emp where empno=’7900′; 会取出所有的字段；生产中杜绝应用

起因 1：占用更多的内存空间（缓冲池 innodb buffer pool）

起因 2：查问条件是一般索引，就会进行回表查问（随机 io，查问性能升高）

。select col_name from ..

select ename, job from emp where empno=’7900′;

索引笼罩（using index）

。select col_name as new_col_name from ..

select sum(sal) as t_sal from emp; 取别名

。select distinct col_name from ..

select distinct job from emp; 去重

。作用：用于条件过滤，有条件的从表中获取数据

。构造：where col_name 操作符值

where name = ‘ergou’;

。where 子句中的操作符

	罕用操作符
	< , <= , > , >= , = , !=
	between..and..
	in(set)
	like
	not like
	is null
	and
	or
	not

范畴操作符：< , <= , > , >= , between..and..，in(set)，not in(set)

select * from emp where empno > 7900;

select * from emp where empno >= 7900;

select * from emp where empno between 7900 and 9999; 查问后果蕴含两边的值

select * from emp where empno in(7900,7902);

select * from emp where empno not in(7900,7902); 在生产中杜绝应用；

等值查问：=

select * from emp where ename = ‘smith’;

注意事项：

字符串类型、日期类型的 sql 语句 where 条件中，肯定要加引号，否则将会产生隐式转换，将进行全表扫描，效率极低。

explain select * from emp where ename = ‘007’; 应用到索引，索引利用级别为 type:ref

explain select * from emp where ename = 007; 没有应用到索引，将会进行全表扫描，sql 效率非常低

不等值查问：!=

生产中不倡议应用；如果是一般索引作为查问条件，将进行全表扫描；

select * from emp where empno != 7900;

含糊查问：like，not like

select * from emp where ename like ‘s%’;

explain select * from emp where ename like ‘%n’; 生产中杜绝应用右边含糊查问

select * from emp where mgr is null;

逻辑操作符：and or not

select ename from emp where empno > 7900 and job= ‘clerk’;

select ename from emp where empno=7934 or empno=9999;

。where 子句查问案例

查问工资高于 2000 的员工

select * from emp where sal > 2000;

查问工资在 2000 到 3500 的员工状况

select * from emp where sal between 2000 and 3500;

查问员工姓名首字母为 S 的员工姓名和工资

select ename ,sal from emp where ename like ‘s%’;

查问 empno 为 7839,7902,7934 的员工状况

select * from emp where empno in(7839,7902,7934);

查问没有下级的员工状况

select * from emp where mgr is null;

查问工资高于 500 或是岗位为 MANAGER，同时还要满足他们的姓名首写字母为 J 的员工信息

select * from emp where (sal > 500 or job = ‘MANAGER’) and ename like ‘j%’;

。作用：对查问后果进行分组统计,having 用于限度分组后果的显示

。构造：group by col_name1, col_name2 having ..

。注意事项：select @@sql_mode; ONLY_FULL_GROUP_BY

查问列必须是 group by 前面的字段或者是聚合函数，呈现其余列将报错

。group by 子句查问案例

查问每个部门的平均工资和最高工资

select deptno,avg(sal), max(sal) from emp group by deptno;

查问每个部门的每种岗位的平均工资和最低工资

select deptno, job, avg(sal),min(sal) from emp group by deptno,job;

查问部门平均工资低于 2000 的部门号和平均工资

select deptno,avg(sal) from emp group by deptno having avg(sal) > 2000;

。作用：用于排序

。构造：order by col_name [asc | desc];

select sal from emp order by sal ;

。注意事项：排序字段是索引，如果指定排序为降序，无奈应用索引的排序，须要从新排序

。order by 子句案例

查问工资的从低到高的程序显示员工的信息

select * from emp order by sal asc;

查问依照员工的年工资进行降序排序显示员工的姓名和年工资

select ename , sal*12 as y_sal from emp order by y_sal desc;

。作用：用于限度取出的记录数，也能够用于分页

select * from emp limit 2;

分页：limit (pagenow-1)*pagesize,pagesize

。分页查问案例：

按员工号升序查问员工姓名、工资，每页显示 3 条记录。请别离显示第一页，第二页，第三页

select ename,sal from emp order by empno asc limit 0,3;

select ename,sal from emp order by empno asc limit 3,3;

select ename,sal from emp order by empno asc limit 6,3;

关于mysql:MySQL-select查询教程

查问列

where 子句

group by .. having 子句

order by 子句

limit 子句