深入浅出MySQL(3)
1.distinct关键字可使得表中的记录去掉重复后显示出来:
mysql> select * from emp;
+------+--------+-------+------------+-----------+--------+
| age1 | ename | birth | hiredate | sal | deptno |
+------+--------+-------+------------+-----------+--------+
| NULL | zzx1 | NULL | 2000-01-01 | 200000.00 | 1 |
| NULL | lisa | NULL | 2003-02-01 | 4000.00 | 2 |
| NULL | bjguan | NULL | 2004-04-02 | 5000.00 | 3 |
| NULL | bjguan | NULL | 2004-04-02 | 5000.00 | 1 |
+------+--------+-------+------------+-----------+--------+
4 rows in set (0.00 sec)
mysql> select distinct deptno from emp;
+--------+
| deptno |
+--------+
| 1 |
| 2 |
| 3 |
+--------+
3 rows in set (0.06 sec)用where实现限定条件查询:
mysql> select * from emp where deptno = 1;
+------+--------+-------+------------+-----------+--------+
| age1 | ename | birth | hiredate | sal | deptno |
+------+--------+-------+------------+-----------+--------+
| NULL | zzx1 | NULL | 2000-01-01 | 200000.00 | 1 |
| NULL | bjguan | NULL | 2004-04-02 | 5000.00 | 1 |
+------+--------+-------+------------+-----------+--------+
2 rows in set (0.08 sec)ORDER BY实现数据库排序操作，语法:
SELECT * FROM tablename [WHERE CONDITION][ORDER BY field1[DESC|ASC]field2[DESC|ASC],……fieldn[DESC|ASC]]
DESC指按降序排列，ASC为按升序排列，如将emp中的sal由低至高进行排序:
mysql> select * from emp order by sal;
+------+--------+-------+------------+-----------+--------+
| age1 | ename | birth | hiredate | sal | deptno |
+------+--------+-------+------------+-----------+--------+
| NULL | lisa | NULL | 2003-02-01 | 4000.00 | 2 |
| NULL | bjguan | NULL | 2004-04-02 | 5000.00 | 3 |
| NULL | bjguan | NULL | 2004-04-02 | 5000.00 | 1 |
| NULL | zzx1 | NULL | 2000-01-01 | 200000.00 | 1 |
+------+--------+-------+------------+-----------+--------+
4 rows in set (0.00 sec)对于deptno相同的前两条记录，按照工资由高到低排序:
mysql> select * from emp order by deptno,sal desc;
+------+--------+-------+------------+-----------+--------+
| age1 | ename | birth | hiredate | sal | deptno |
+------+--------+-------+------------+-----------+--------+
| NULL | zzx1 | NULL | 2000-01-01 | 200000.00 | 1 |
| NULL | bjguan | NULL | 2004-04-02 | 5000.00 | 1 |
| NULL | lisa | NULL | 2003-02-01 | 4000.00 | 2 |
| NULL | bjguan | NULL | 2004-04-02 | 5000.00 | 3 |
+------+--------+-------+------------+-----------+--------+
4 rows in set (0.00 sec)对于记录如果只想显示一部分的话，用LIMIT实现，语法:SELECT …… [LIMIT offset_start,row_count]
offset_start表示起始偏移量，row_count表示显示的行数，显示按sal排序的前3条记录:
mysql> select * from emp order by sal limit 3;
+------+--------+-------+------------+---------+--------+
| age1 | ename | birth | hiredate | sal | deptno |
+------+--------+-------+------------+---------+--------+
| NULL | lisa | NULL | 2003-02-01 | 4000.00 | 2 |
| NULL | bjguan | NULL | 2004-04-02 | 5000.00 | 3 |
| NULL | bjguan | NULL | 2004-04-02 | 5000.00 | 1 |
+------+--------+-------+------------+---------+--------+
3 rows in set (0.00 sec)要从第二条记录开始显示的话:
mysql> select * from emp order by sal limit 1,3;
+------+--------+-------+------------+-----------+--------+
| age1 | ename | birth | hiredate | sal | deptno |
+------+--------+-------+------------+-----------+--------+
| NULL | bjguan | NULL | 2004-04-02 | 5000.00 | 3 |
| NULL | bjguan | NULL | 2004-04-02 | 5000.00 | 1 |
| NULL | zzx1 | NULL | 2000-01-01 | 200000.00 | 1 |
+------+--------+-------+------------+-----------+--------+
3 rows in set (0.00 sec)2.对SQL进行聚合操作的语法:
SELECT[field1,field2,……fieldn]fun_name
FROM tablename
[WHERE where_condition]
[GROUP BY field1,field2,……fieldn
[WITH POLLUP]
[HAVING where_condition]
fun_name为聚合函数，如sum,count(*)(记录函数),max,min.
GROUP BY 表示要进行分类聚合的字段
WITH ROLLUP 是否对分类聚合后的结果再汇总.
HAVING 对分类后的结果进行条件的过滤(having为对聚合后的结果进行过滤，where为对聚合前的结果进行过滤)
统计emp表中的人数进行统计:
mysql> select count(1) from emp;
+----------+
| count(1) |
+----------+
| 4 |
+----------+
1 row in set (0.10 sec)统计各个部门人数:
mysql> select deptno,count(1) from emp group by deptno;
+--------+----------+
| deptno | count(1) |
+--------+----------+
| 1 | 2 |
| 2 | 1 |
| 3 | 1 |
+--------+----------+
3 rows in set (0.06 sec)既统计各部门人数，又统计总人数:
mysql> select deptno,count(1) from emp group by deptno with rollup;
+--------+----------+
| deptno | count(1) |
+--------+----------+
| 1 | 2 |
| 2 | 1 |
| 3 | 1 |
| NULL | 4 |
+--------+----------+
4 rows in set (0.00 sec)统计人数大于1的部门:
mysql> select deptno,count(1) from emp group by deptno having count(1)>1;
+--------+----------+
| deptno | count(1) |
+--------+----------+
| 1 | 2 |
+--------+----------+
1 row in set (0.07 sec)统计所有员工sal总额，最高和最低sal:
mysql> select sum(sal),max(sal),min(sal) from emp;
+-----------+-----------+----------+
| sum(sal) | max(sal) | min(sal) |
+-----------+-----------+----------+
| 214000.00 | 200000.00 | 4000.00 |
+-----------+-----------+----------+
1 row in set (0.03 sec)将emp和dept进行内连接，查出雇员的名字和所在的部门:
mysql> select * from dept;
+--------+----------+
| deptno | deptname |
+--------+----------+
| 1 | tech |
| 2 | sale |
| 3 | hr |
+--------+----------+
3 rows in set (0.00 sec)
mysql> select ename,deptname from emp,dept where emp.deptno = dept.deptno;
+--------+----------+
| ename | deptname |
+--------+----------+
| zzx1 | tech |
| lisa | sale |
| bjguan | hr |
| bjguan | tech |
+--------+----------+
4 rows in set (0.10 sec)