1.简单的查询语句(DQL):
select 字段名1,字段名2,字段名3,.... from 表名;
注意: 1、任何一条sql语句都以“;”结尾; 2、sql语句不区分大小写; 查询员工的年薪?(字段可以参与数学运算) select ename,sal * 12 from emp; +--------+----------+ | ename | sal * 12 | +--------+----------+ | SMITH | 9600.00 | | ALLEN | 19200.00 | | WARD | 15000.00 | | JONES | 35700.00 | | MARTIN | 15000.00 | | BLAKE | 34200.00 | | CLARK | 29400.00 | | SCOTT | 36000.00 | | KING | 60000.00 | | TURNER | 18000.00 | | ADAMS | 13200.00 | | JAMES | 11400.00 | | FORD | 36000.00 | | MILLER | 15600.00 | +--------+----------+ 给查询结果的列重命名? select ename,sal * 12 as yearsal from emp; 别名中有中文? select ename,sal * 12 as 年薪 from emp; // 错误 select ename,sal * 12 as \'年薪\' from emp; // 正确 +--------+----------+ | ename | 年薪 | +--------+----------+ | SMITH | 9600.00 | | ALLEN | 19200.00 | | WARD | 15000.00 | | JONES | 35700.00 | | MARTIN | 15000.00 | | BLAKE | 34200.00 | | CLARK | 29400.00 | | SCOTT | 36000.00 | | KING | 60000.00 | | TURNER | 18000.00 | | ADAMS | 13200.00 | | JAMES | 11400.00 | | FORD | 36000.00 | | MILLER | 15600.00 | +--------+----------+ 注意:标准sql语句中要求字符串使用单引号括起来,虽然mysql支持双引号,尽量别用;
as 关键字可以省略;
查询所有字段? select * from emp; // 实际开发中不建议使用*,效率较低
2.条件查询:
查询工资等于5000的员工姓名? select ename from emp where sal = 5000; +-------+ | ename | +-------+ | KING | +-------+ 查询SMITH的工资? select sal from emp where ename = \'SMITH\'; // 字符串使用单引号括起来。 +--------+ | sal | +--------+ | 800.00 | +--------+ 找出工资高于3000的员工? select ename,sal from emp where sal > 3000;
类比:
select ename,sal from emp where sal >= 3000;
select ename,sal from emp where sal < 3000;
select ename,sal from emp where sal <= 3000; select ename,sal from emp where sal <> 3000;
select ename,sal from emp where sal != 3000; 找出工资在1100和3000之间的员工(包括1100和3000)? select ename,sal from emp where sal >= 1100 and sal <= 3000;
select ename,sal from emp where sal between 1100 and 3000; // between...and...是闭区间 [1100 ~ 3000]
select ename,sal from emp where sal between 3000 and 1100;
// 查询不到任何数据(between and在使用的时候必须左小右大)
between and除了可以使用在数字方面之外,还可以使用在字符串方面: select ename from emp where ename between \'A\' and \'C\'; // 左闭右开 +-------+ | ename | +-------+ | ALLEN | | BLAKE | | ADAMS | +-------
找出哪些人津贴为NULL? 在数据库当中NULL不是一个值,代表什么也没有,为空。 空不是一个值,不能用等号衡量;必须使用 is null或者is not null select ename,sal,comm from emp where comm is null; +--------+---------+------+ | ename | sal | comm | +--------+---------+------+ | SMITH | 800.00 | NULL | | JONES | 2975.00 | NULL | | BLAKE | 2850.00 | NULL | | CLARK | 2450.00 | NULL | | SCOTT | 3000.00 | NULL | | KING | 5000.00 | NULL | | ADAMS | 1100.00 | NULL | | JAMES | 950.00 | NULL | | FORD | 3000.00 | NULL | | MILLER | 1300.00 | NULL | +--------+---------+------+ select ename,sal,comm from emp where comm = null; Empty set (0.00 sec)
找出哪些人津贴不为NULL? select ename,sal,comm from emp where comm is not null; +--------+---------+---------+ | ename | sal | comm | +--------+---------+---------+ | ALLEN | 1600.00 | 300.00 | | WARD | 1250.00 | 500.00 | | MARTIN | 1250.00 | 1400.00 | | TURNER | 1500.00 | 0.00 | +--------+---------+---------+ 找出哪些人没有津贴? select ename,sal,comm from emp where comm is null or comm = 0; +--------+---------+------+ | ename | sal | comm | +--------+---------+------+ | SMITH | 800.00 | NULL | | JONES | 2975.00 | NULL | | BLAKE | 2850.00 | NULL | | CLARK | 2450.00 | NULL | | SCOTT | 3000.00 | NULL | | KING | 5000.00 | NULL | | TURNER | 1500.00 | 0.00 | | ADAMS | 1100.00 | NULL | | JAMES | 950.00 | NULL | | FORD | 3000.00 | NULL | | MILLER | 1300.00 | NULL | +--------+---------+------+ 找出工作岗位是MANAGER和SALESMAN的员工? select ename,job from emp where job = \'MANAGER\' or job = \'SALESMAN\'; +--------+----------+ | ename | job | +--------+----------+ | ALLEN | SALESMAN | | WARD | SALESMAN | | JONES | MANAGER | | MARTIN | SALESMAN | | BLAKE | MANAGER | | CLARK | MANAGER | | TURNER | SALESMAN | +--------+----------+ and和or联合起来用:找出薪资大于1000的并且部门编号是20或30部门的员工。 select ename,sal,deptno from emp where sal > 1000 and deptno = 20 or deptno = 30; // 错误 select ename,sal,deptno from emp where sal > 1000 and (deptno = 20 or deptno = 30);
// 正确(注意:当运算符的优先级不确定的时候加小括号)
in等同于or:找出工作岗位是MANAGER和SALESMAN的员工? select ename,job from emp where job = \'SALESMAN\' or job = \'MANAGER\'; select ename,job from emp where job in(\'SALESMAN\', \'MANAGER\'); // 与上一行语句等价
select ename,job from emp where sal in(800, 5000); // in后面的值不是区间,是具体的值 not in: 不在这几个值当中: select ename,job from emp where sal not in(800, 5000); 模糊查询like ? (针对字符串) 找出名字当中含有O的?(在模糊查询当中,必须掌握两个特殊的符号,一个是%,一个是_) %代表任意多个字符,_代表任意1个字符。 select ename from emp where ename like \'%O%\'; +-------+ | ename | +-------+ | JONES | | SCOTT | | FORD | +-------+ 找出名字中第二个字母是A的? select ename from emp where ename like \'_A%\'; +--------+ | ename | +--------+ | WARD | | MARTIN | | JAMES | +--------+ 找出名字中有下划线的?
select name from t_user where name like \'%_%\';(错误) +----------+ | name | +----------+ | zhangsan | | lisi | | WANG_WU | +----------+ select name from t_user where name like \'%\\_%\';(正确) +---------+ | name | +---------+ | WANG_WU | +---------+
找出名字中最后一个字母是T的? select ename from emp where ename like \'%T\'; +-------+ | ename | +-------+ | SCOTT | +-------+
3. 排序(升序、降序)
按照工资升序,找出员工名和薪资? select ename,sal from emp order by sal;+--------+---------+| ename | sal |+--------+---------+| SMITH | 800.00 || JAMES | 950.00 || ADAMS | 1100.00 || WARD | 1250.00 || MARTIN | 1250.00 || MILLER | 1300.00 || TURNER | 1500.00 || ALLEN | 1600.00 || CLARK | 2450.00 || BLAKE | 2850.00 || JONES | 2975.00 || FORD | 3000.00 || SCOTT | 3000.00 || KING | 5000.00 |+--------+---------+
注意:默认是升序。怎么指定升序或者降序呢?asc表示升序,desc表示降序 select ename , sal from emp order by sal; // 升序 select ename , sal from emp order by sal asc; // 升序 select ename , sal from emp order by sal desc; // 降序
按照工资的降序排列,当工资相同的时候再按照名字的升序排列。 select ename,sal from emp order by sal desc; select ename,sal from emp order by sal desc , ename asc; 注意:越靠前的字段越能起到主导作用。只有当前面的字段无法完成排序的时候,才会启用后面的字段。
找出工作岗位是SALESMAN的员工,并且要求按照薪资的降序排列。 select ename,job,sal from emp where job = \'SALESMAN\' order by sal desc;+--------+----------+---------+| ename | job | sal |+--------+----------+---------+| ALLEN | SALESMAN | 1600.00 || TURNER | SALESMAN | 1500.00 || WARD | SALESMAN | 1250.00 || MARTIN | SALESMAN | 1250.00 |+--------+----------+---------+
4. 分组函数:所有的分组函数都是对“某一组”数据进行操作的;
count 计数 sum 求和 avg 平均值 max 最大值 min 最小值
找出工资总和? select sum(sal) from emp; 找出最高工资? select max(sal) from emp; 找出最低工资? select min(sal) from emp; 找出平均工资? select avg(sal) from emp; 找出总人数? select count(*) from emp; select count(ename) from emp;
count(*)和count(具体的某个字段),他们有什么区别? count(*):不是统计某个字段中数据的个数,而是统计总记录条数。(和某个字段无关) count(comm): 表示统计comm字段中不为NULL的数据总数量。 分组函数还有另一个名字:多行处理函数;多行处理函数的特点:输入多行,最终输出的结果是1行。
分组函数自动忽略NULL。 select count(comm) from emp; +-------------+ | count(comm) | +-------------+ | 4 | +-------------+
select sum(comm) from emp; +-----------+ | sum(comm) | +-----------+ | 2200.00 | +-----------+
select sum(comm) from emp where comm is not null; // 不需要额外添加这个过滤条件。sum函数自动忽略NULL。
找出工资高于平均工资的员工?
select ename,sal from emp where sal > avg(sal); //ERROR 1111 (HY000): Invalid use of group function 思考以上的错误信息:无效的使用了分组函数? 原因:SQL语句当中有一个语法规则,分组函数不可直接使用在where子句当中。why???? 怎么解释? group by是在where执行之后才会执行的!
分组函数也能组合起来用: select count(*),sum(sal),avg(sal),max(sal),min(sal) from emp;
+----------+----------+-------------+----------+----------+ | count(*) | sum(sal) | avg(sal) | max(sal) | min(sal) | +----------+----------+-------------+----------+----------+ | 14 | 29025.00 | 2073.214286 | 5000.00 | 800.00 | +----------+----------+-------------+----------+----------+ 找出工资高于平均工资的员工? 第一步:找出平均工资 select avg(sal) from emp; +-------------+ | avg(sal) | +-------------+ | 2073.214286 | +-------------+ 第二步:找出高于平均工资的员工 select ename,sal from emp where sal > 2073.214286; +-------+---------+ | ename | sal | +-------+---------+ | JONES | 2975.00 | | BLAKE | 2850.00 | | CLARK | 2450.00 | | SCOTT | 3000.00 | | KING | 5000.00 | | FORD | 3000.00 | +-------+---------+
合体版:select ename,sal from emp where sal > (select avg(sal) from emp);
5.单行处理函数:输入一行,输出一行
计算每个员工的年薪? select ename,(sal+comm)*12 as yearsal from emp; 重点:所有数据库都是这样规定的,只要有NULL参与的运算结果一定是NULL。 使用ifnull函数: select ename,(sal+ifnull(comm,0))*12 as yearsal from emp; ifnull() 空处理函数: ifnull(可能为NULL的数据,被当做什么处理) : 属于单行处理函数。 select ename,ifnull(comm,0) as comm from emp; +--------+---------+ | ename | comm | +--------+---------+ | SMITH | 0.00 | | ALLEN | 300.00 | | WARD | 500.00 | | JONES | 0.00 | | MARTIN | 1400.00 | | BLAKE | 0.00 | | CLARK | 0.00 | | SCOTT | 0.00 | | KING | 0.00 | | TURNER | 0.00 | | ADAMS | 0.00 | | JAMES | 0.00 | | FORD | 0.00 | | MILLER | 0.00 | +--------+---------+
6. group by 和 having
group by : 按照某个字段或者某些字段进行分组。 having : having是对分组之后的数据进行再次过滤。
案例:找出每个工作岗位的最高薪资。 select max(sal),job from emp group by job;
+----------+-----------+ | max(sal) | job | +----------+-----------+ | 3000.00 | ANALYST | | 1300.00 | CLERK | | 2975.00 | MANAGER | | 5000.00 | PRESIDENT | | 1600.00 | SALESMAN | +----------+-----------+
注意:分组函数一般都会和group by联合使用,这也是为什么它被称为分组函数的原因,并且任何一个分组函数(count sum avg max min)都是在group by语句执行结束之后才会执行的,当一条sql语句没有group by的话,整张表的数据会自成一组。
select ename,max(sal),job from emp group by job; 以上在mysql当中,查询结果是有的,但是结果没有意义(在Oracle数据库当中会报错。语法错误); 记住一个规则:当一条语句中有group by的话,select后面只能跟分组函数和参与分组的字段。
每个工作岗位的平均薪资? select job,avg(sal) from emp group by job; +-----------+-------------+ | job | avg(sal) | +-----------+-------------+ | ANALYST | 3000.000000 | | CLERK | 1037.500000 | | MANAGER | 2758.333333 | | PRESIDENT | 5000.000000 | | SALESMAN | 1400.000000 | +-----------+-------------+
找出每个部门的最高薪资,要求显示薪资大于2900的数据。
第一步:找出每个部门的最高薪资 select max(sal),deptno from emp group by deptno; +----------+--------+ | max(sal) | deptno | +----------+--------+ | 5000.00 | 10 | | 3000.00 | 20 | | 2850.00 | 30 | +----------+--------+
第二步:找出薪资大于2900 select max(sal),deptno from emp group by deptno having max(sal) > 2900; // 这种方式效率低 +----------+--------+ | max(sal) | deptno | +----------+--------+ | 5000.00 | 10 | | 3000.00 | 20 | +----------+--------+
合体版:select max(sal),deptno from emp where sal > 2900 group by deptno;
// 效率较高,建议能够使用where过滤的尽量使用where +----------+--------+ | max(sal) | deptno | +----------+--------+ | 5000.00 | 10 | | 3000.00 | 20 | +----------+--------+ 找出每个部门的平均薪资,要求显示薪资大于2000的数据。
第一步:找出每个部门的平均薪资 select deptno,avg(sal) from emp group by deptno; +--------+-------------+ | deptno | avg(sal) | +--------+-------------+ | 10 | 2916.666667 | | 20 | 2175.000000 | | 30 | 1566.666667 | +--------+-------------+
第二步:要求显示薪资大于2000的数据 select deptno,avg(sal) from emp group by deptno having avg(sal) > 2000; +--------+-------------+ | deptno | avg(sal) | +--------+-------------+ | 10 | 2916.666667 | | 20 | 2175.000000 | +--------+-------------+
where后面不能使用分组函数: select deptno,avg(sal) from emp where avg(sal) > 2000 group by deptno; //错误,所以这种情况只能使用having过滤;
关于查询结果集的去重? mysql> select distinct job from emp; // distinct关键字去除重复记录。 +-----------+ | job | +-----------+ | CLERK | | SALESMAN | | MANAGER | | ANALYST | | PRESIDENT | +-----------+
mysql> select ename,distinct job from emp;
//以上的sql语句是错误的,记住:distinct只能出现在所有字段的最前面
mysql> select distinct deptno,job from emp; +--------+-----------+ | deptno | job | +--------+-----------+ | 20 | CLERK | | 30 | SALESMAN | | 20 | MANAGER | | 30 | MANAGER | | 10 | MANAGER | | 20 | ANALYST | | 10 | PRESIDENT | | 30 | CLERK | | 10 | CLERK | +--------+-----------+
案例:统计岗位的数量? select count(distinct job) from emp;
+---------------------+ | count(distinct job) | +---------------------+ | 5 | +---------------------+
6. 目前SQL语句执行顺序:
select (5) .. from (1) .. where (2) .. group by (3) .. having (4) .. order by (6) .. ;
资料出处:https://www.bilibili.com/video/BV1fx411X7BD
来源:https://www.cnblogs.com/Burning-youth/p/15676723.html
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