1.查询语言：（以当前表名为emp为例）
（1）简单查询
查询所有的列//create * from 表名；
mysql> select * from emp;
+----+----------+--------+------------+----------+------------+----------+
| id | name | gender | birthday | salary | entry_date | resume |
+----+----------+--------+------------+----------+------------+----------+
| 1 | zhangsan | female | 1990-05-10 | 10000.00 | 2015-05-05 | goodgirl |
| 2 | lisi | male | 1995-05-10 | 10000.00 | 2015-05-05 | good boy |
| 3 | 你好 | male | 1995-05-10 | 10000.00 | 2015-05-05 | good boy |
+----+----------+--------+------------+----------+------------+----------+
3 rows in set (0.00 sec)查询指定列//select name，birthday from emp；
mysql> select name,birthday from emp;
+----------+------------+
| name | birthday |
+----------+------------+
| zhangsan | 1990-05-10 |
| lisi | 1995-05-10 |
| 你好 | 1995-05-10 |
+----------+------------+
3 rows in set (0.00 sec)（2）条件查询
条件查询就是在查询时给出where子句，在where子句中可以使用如下运算符和关键字：
1.运算符（=、！=、<、>、<=、>=、<>）
2.关键字
between ..and...;例查询1是否在0和3之间
mysql> select 1 between 0 and 3;
+-------------------+
| 1 between 0 and 3 |
+-------------------+
| 1 |/*！1 显示为true*/
+-------------------+
1 row in set (0.00 sec)in（set）；//判断是否在这个集合中
mysql> select 2 in(0,2);//2为该集合中的元素
+-----------+
| 2 in(0,2) |
+-----------+
| 1 |
+-----------+
1 row in set (0.00 sec)is null；//例查询emp中name为null的记录
mysql> select * from emp where name is null;//当前显示为name没有为null的
Empty set (0.00 sec)
and；&；|；！ 的使用
mysql> select * from emp where salary>800 and salary<10000;
+----+-------+--------+------------+---------+------------+----------+
| id | name | gender | birthday | salary | entry_date | resume |
+----+-------+--------+------------+---------+------------+----------+
| 3 | lihua | male | 1995-05-10 | 1800.00 | 2015-05-05 | good boy |
+----+-------+--------+------------+---------+------------+----------+
1 row in set (0.00 sec)
mysql> select * from emp where resume!='goodgirl';
+----+-------+--------+------------+----------+------------+----------+
| id | name | gender | birthday | salary | entry_date | resume |
+----+-------+--------+------------+----------+------------+----------+
| 2 | lisi | male | 1995-05-10 | 10000.00 | 2015-05-05 | good boy |
| 3 | lihua | male | 1995-05-10 | 1800.00 | 2015-05-05 | good boy |
+----+-------+--------+------------+----------+------------+----------+
2 rows in set (0.00 sec)
mysql> select * from emp where resume<>'goodgirl';
+----+-------+--------+------------+----------+------------+----------+
| id | name | gender | birthday | salary | entry_date | resume |
+----+-------+--------+------------+----------+------------+----------+
| 2 | lisi | male | 1995-05-10 | 10000.00 | 2015-05-05 | good boy |
| 3 | lihua | male | 1995-05-10 | 1800.00 | 2015-05-05 | good boy |
+----+-------+--------+------------+----------+------------+----------+
2 rows in set (0.00 sec)
（3）模糊查询
当想查询姓名中包含a字母的人时就需要使用模糊查询了。模糊查询需要使用关键字like。
通配符：_ :代表一位 %：代表多位
mysql> select * from emp where name like '_i%';
+----+-------+--------+------------+----------+------------+----------+
| id | name | gender | birthday | salary | entry_date | resume |
+----+-------+--------+------------+----------+------------+----------+
| 2 | lisi | male | 1995-05-10 | 10000.00 | 2015-05-05 | good boy |
| 3 | lihua | male | 1995-05-10 | 1800.00 | 2015-05-05 | good boy |
+----+-------+--------+------------+----------+------------+----------+
2 rows in set (0.00 sec)
mysql> select * from emp where name like 'z%';
+----+----------+--------+------------+--------+------------+----------+
| id | name | gender | birthday | salary | entry_date | resume |
+----+----------+--------+------------+--------+------------+----------+
| 1 | zhangsan | female | 1990-05-10 | 800.00 | 2015-05-05 | goodgirl |
+----+----------+--------+------------+--------+------------+----------+
1 row in set (0.00 sec)
（4）字段控制查询
1.去除重复记录（两行或两行以上记录系列上的数据都相同），例上表中的resume中的goodboy，当查询emp表的resume字段时，那么会重复记录，那么想去除重复记录，需要使用distinct：
mysql> select distinct resume from emp;
+----------+
| resume |
+----------+
| goodgirl |
| good boy |
+----------+
2 rows in set (0.00 sec)2.先将上表中添加一列comm默认值为1000；查看salary列和comm列之和。因为salary和comm列都为int型，所以可以做加法运算，如果其中一列不是int型，那么会出错。
mysql> select *,salary+comm from emp;
+----+----------+--------+------------+----------+------------+----------+------+-------------+
| id | name | gender | birthday | salary | entry_date | resume | comm | salary+comm |
+----+----------+--------+------------+----------+------------+----------+------+-------------+
| 1 | zhangsan | female | 1990-05-10 | 800.00 | 2015-05-05 | goodgirl | 1000 | 1800.00 |
| 2 | lisi | male | 1995-05-10 | 10000.00 | 2015-05-05 | good boy | 1000 | 11000.00 |
| 3 | lihua | male | 1995-05-10 | 1800.00 | 2015-05-05 | good boy | 1000 | 2800.00 |
+----+----------+--------+------------+----------+------------+----------+------+-------------+
3 rows in set (0.00 sec)3.给列名添加别名（将上表中的salary+comm列更名为num） 其中as 可以省略
mysql> select *,salary+comm as num from emp;
+----+----------+--------+------------+----------+------------+----------+------+----------+
| id | name | gender | birthday | salary | entry_date | resume | comm | num |
+----+----------+--------+------------+----------+------------+----------+------+----------+
| 1 | zhangsan | female | 1990-05-10 | 800.00 | 2015-05-05 | goodgirl | 1000 | 1800.00 |
| 2 | lisi | male | 1995-05-10 | 10000.00 | 2015-05-05 | good boy | 1000 | 11000.00 |
| 3 | lihua | male | 1995-05-10 | 1800.00 | 2015-05-05 | good boy | 1000 | 2800.00 |
+----+----------+--------+------------+----------+------------+----------+------+----------+
3 rows in set (0.00 sec)（5）排序
1.查询salary列 ，按数值从小到大排列（select * from emp order by 列名 asc； ）
mysql> select * from emp order by salary asc;
+----+----------+--------+------------+----------+------------+----------+------+
| id | name | gender | birthday | salary | entry_date | resume | comm |
+----+----------+--------+------------+----------+------------+----------+------+
| 1 | zhangsan | female | 1990-05-10 | 800.00 | 2015-05-05 | goodgirl | 1000 |
| 3 | lihua | male | 1995-05-10 | 1800.00 | 2015-05-05 | good boy | 1000 |
| 2 | lisi | male | 1995-05-10 | 10000.00 | 2015-05-05 | good boy | 1000 |
+----+----------+--------+------------+----------+------------+----------+------+
3 rows in set (0.00 sec)或者 （select * from emp order by salary；）
mysql> select * from emp order by salary;
+----+----------+--------+------------+----------+------------+----------+------+
| id | name | gender | birthday | salary | entry_date | resume | comm |
+----+----------+--------+------------+----------+------------+----------+------+
| 1 | zhangsan | female | 1990-05-10 | 800.00 | 2015-05-05 | goodgirl | 1000 |
| 3 | lihua | male | 1995-05-10 | 1800.00 | 2015-05-05 | good boy | 1000 |
| 2 | lisi | male | 1995-05-10 | 10000.00 | 2015-05-05 | good boy | 1000 |
+----+----------+--------+------------+----------+------------+----------+------+
3 rows in set (0.00 sec)2.按salary列的从大到小排列（select * from emp order by desc；）
mysql> select * from emp order by salary desc;
+----+----------+--------+------------+----------+------------+----------+------+
| id | name | gender | birthday | salary | entry_date | resume | comm |
+----+----------+--------+------------+----------+------------+----------+------+
| 2 | lisi | male | 1995-05-10 | 10000.00 | 2015-05-05 | good boy | 1000 |
| 3 | lihua | male | 1995-05-10 | 1800.00 | 2015-05-05 | good boy | 1000 |
| 1 | zhangsan | female | 1990-05-10 | 800.00 | 2015-05-05 | goodgirl | 1000 |
+----+----------+--------+------------+----------+------------+----------+------+
3 rows in set (0.00 sec)3.查看所有人，按照salary列从大到小排序，如果 resume列相同，则按name列排序（select * from emp where resume=’goodboy‘ order by name desc；）
mysql> select * from emp where resume='good boy' order by name desc;//降序
+----+-------+--------+------------+----------+------------+----------+------+
| id | name | gender | birthday | salary | entry_date | resume | comm |
+----+-------+--------+------------+----------+------------+----------+------+
| 2 | lisi | male | 1995-05-10 | 10000.00 | 2015-05-05 | good boy | 1000 |
| 3 | lihua | male | 1995-05-10 | 1800.00 | 2015-05-05 | good boy | 1000 |
+----+-------+--------+------------+----------+------------+----------+------+
2 rows in set (0.00 sec)
mysql> select * from emp where resume='good boy' order by name ;//升序
+----+-------+--------+------------+----------+------------+----------+------+
| id | name | gender | birthday | salary | entry_date | resume | comm |
+----+-------+--------+------------+----------+------------+----------+------+
| 3 | lihua | male | 1995-05-10 | 1800.00 | 2015-05-05 | good boy | 1000 |
| 2 | lisi | male | 1995-05-10 | 10000.00 | 2015-05-05 | good boy | 1000 |
+----+-------+--------+------------+----------+------------+----------+------+
2 rows in set (0.00 sec)（6）聚合函数：聚合函数都只有一个结果
聚合函数是用来做做纵向运算的函数：
1.count（）：统计指定列不为null的记录行数；
2.max（）：计算指定列的最大值，如果指定列为字符串类型，那么使用字符串排序运算；
3.min（）：计算指定列的最小值，如果指定列为字符串类型，那么使用字符串排序运算；
4.sum（）：计算指定列的数值和，如果指定列类型不是数值类型，那么计算结果为0；
5.avg（）：计算指定列的平均值，如果指定列的类型不是数值类型，那么计算结果为0；
代码如下：
mysql> select count(*) as name from emp;//count（）统计指定列不为null的个数
+------+
| name |
+------+
| 3 |
+------+
1 row in set (0.00 sec)
mysql> select max(salary) from emp;//用max（） 找salary列中的最大值
+-------------+
| max(salary) |
+-------------+
| 10000.00 |
+-------------+
1 row in set (0.00 sec)
mysql> select min(salary) from emp;//min（） 找salary列的最小值
+-------------+
| min(salary) |
+-------------+
| 800.00 |
+-------------+
1 row in set (0.00 sec)
mysql> select sum(salary) from emp;//求salary列的和
+-------------+
| sum(salary) |
+-------------+
| 12600.00 |
+-------------+
1 row in set (0.00 sec)
mysql> select avg(salary) from emp;//求平均
+-------------+
| avg(salary) |
+-------------+
| 4200.000000 |
+-------------+
1 row in set (0.00 sec)（7）分组查询（当前使用新的emp表）表格如下
mysql> select * from emp;
+-------+--------+-----------+------+------------+---------+---------+--------+
| empno | ename | job | mgr | hiredate | sal | comm | deptno |
+-------+--------+-----------+------+------------+---------+---------+--------+
| 7369 | SMITH | CLERK | 7902 | 1980-12-17 | 800.00 | NULL | 20 |
| 7499 | ALLEN | SALESMAN | 7698 | 1981-02-20 | 1600.00 | 300.00 | 30 |
| 7521 | WARD | SALESMAN | 7698 | 1981-02-22 | 1250.00 | 500.00 | 30 |
| 7566 | JONES | MANAGER | 7839 | 1981-04-02 | 2975.00 | NULL | 20 |
| 7654 | MARTIN | SALESMAN | 7698 | 1981-09-28 | 1250.00 | 1400.00 | 30 |
| 7698 | BLAKE | MANAGER | 7839 | 1981-05-01 | 2850.00 | NULL | 30 |
| 7782 | CLARK | MANAGER | 7839 | 1981-06-09 | 2450.00 | NULL | 10 |
| 7788 | SCOTT | ANALYST | 7566 | 1987-04-19 | 3000.00 | NULL | 20 |
| 7839 | KING | PRESIDENT | NULL | 1981-11-17 | 5000.00 | NULL | 10 |
| 7844 | TURNER | SALESMAN | 7698 | 1981-09-08 | 1500.00 | 0.00 | 30 |
| 7876 | ADAMS | CLERK | 7788 | 1987-05-23 | 1100.00 | NULL | 20 |
| 7900 | JAMES | CLERK | 7698 | 1981-12-03 | 950.00 | NULL | 30 |
| 7902 | FORD | ANALYST | 7566 | 1981-12-03 | 3000.00 | NULL | 20 |
| 7934 | MILLER | CLERK | 7782 | 1982-01-23 | 1300.00 | NULL | 10 |
+-------+--------+-----------+------+------------+---------+---------+--------+
14 rows in set (0.00 sec)当需要分组查询时需要使用group by 子句。
注：凡和聚合函数同时出现的列名，则一定要写在group by 之后
1.查询每个部门（deptno）和每个部门的工资（sal）和。// select deptno，sum（sal）from emp（表名）group by deptno；
mysql> select deptno,sum(sal) from emp group by deptno;/*！按列表的从上往下打印*/
+--------+----------+
| deptno | sum(sal) |
+--------+----------+
| 20 | 10875.00 |
| 30 | 9400.00 |
| 10 | 8750.00 |
+--------+----------+
3 rows in set (0.00 sec)2.查询每个部门的部门编号以及每个部门工资大于1500的人数；//select depyno，count(*） from emp where sal > 1500 group by deptno;
mysql> select deptno,count(*) from emp where sal>1500 group by deptno;
+--------+----------+
| deptno | count(*) |
+--------+----------+
| 30 | 2 |
| 20 | 3 |
| 10 | 2 |
+--------+----------+
3 rows in set (0.00 sec)3.(having)子句
（1）查询工资总和大于9000的部门编号以及工资和；//select deptno，sum（sal） from emp group by deptno having sum（sal）>9000；
mysql> select deptno,sum(sal) from emp group by deptno having sum(sal)>9000;
+--------+----------+
| deptno | sum(sal) |
+--------+----------+
| 20 | 10875.00 |
| 30 | 9400.00 |
+--------+----------+
2 rows in set (0.00 sec)注：where和having的区别：
having ：是在分组后对数据进行过滤，where是在分组前对数据进行过滤
having后面可以使用分组函数（统计函数）
where：where是对分组前记录的条件，如果某行记录没有满足where子句的条件，那么这行记录不会参加分组；而having是对分组后数据的约束。
where后面不可以使用分组函数。
（8）LIMIT
limit用来限定查询结果的起始行，以及总行数。
1.查询emp表5行，起始行从1开始；// select * from emp limit 1,5；
mysql> select * from emp limit 1,5;/*!查询结果从第二行开始向下打印5行*/
+-------+--------+----------+------+------------+---------+---------+--------+
| empno | ename | job | mgr | hiredate | sal | comm | deptno |
+-------+--------+----------+------+------------+---------+---------+--------+
| 7499 | ALLEN | SALESMAN | 7698 | 1981-02-20 | 1600.00 | 300.00 | 30 |
| 7521 | WARD | SALESMAN | 7698 | 1981-02-22 | 1250.00 | 500.00 | 30 |
| 7566 | JONES | MANAGER | 7839 | 1981-04-02 | 2975.00 | NULL | 20 |
| 7654 | MARTIN | SALESMAN | 7698 | 1981-09-28 | 1250.00 | 1400.00 | 30 |
| 7698 | BLAKE | MANAGER | 7839 | 1981-05-01 | 2850.00 | NULL | 30 |
+-------+--------+----------+------+------------+---------+---------+--------+
5 rows in set (0.00 sec)
/*!起始位置为0*/2.分页查询
如果一页记录为4行，那么查询第三页// select * from emp limit 12,4；
mysql> select * from emp limit 12,4;
+-------+--------+---------+------+------------+---------+------+--------+
| empno | ename | job | mgr | hiredate | sal | comm | deptno |
+-------+--------+---------+------+------------+---------+------+--------+
| 7902 | FORD | ANALYST | 7566 | 1981-12-03 | 3000.00 | NULL | 20 |
| 7934 | MILLER | CLERK | 7782 | 1982-01-23 | 1300.00 | NULL | 10 |
+-------+--------+---------+------+------------+---------+------+--------+
2 rows in set (0.00 sec)
/*!因为emp表只有14行，所以第4页只有2行*/(9) 子查询
一个select语句中包含另一个select语句。
子查询是嵌套查询，就是select中包含select，如果一条语句中存两个，或者两个以上的select，那么就是子查询语句了。
子查询出现的位置;
1.where 后，作为被查询条件的一部分；
2.from后，坐表；
当子查询出现在where后作为条件时，还可以使用（any，all）关键字；
子查询结果集的形式;
a.单行单列（用于条件）
b.单行多列（用于条件）
c.多行单列（用于条件）
例：
工资大于jones的工资，其中jones工资需要一条子查询//select * from emp sal >(select sal from emp where ename=’jones‘)；
mysql> select * from emp where sal>(select sal from emp where ename='jones');
+-------+-------+-----------+------+------------+---------+------+--------+
| empno | ename | job | mgr | hiredate | sal | comm | deptno |
+-------+-------+-----------+------+------------+---------+------+--------+
| 7788 | SCOTT | ANALYST | 7566 | 1987-04-19 | 3000.00 | NULL | 20 |
| 7839 | KING | PRESIDENT | NULL | 1981-11-17 | 5000.00 | NULL | 10 |
| 7902 | FORD | ANALYST | 7566 | 1981-12-03 | 3000.00 | NULL | 20 |
+-------+-------+-----------+------+------------+---------+------+--------+
3 rows in set (0.00 sec)单行单列（查询jones的工资）//select sal from emp where ename='jones'；
mysql> select sal from emp where ename='jones';
+---------+
| sal |
+---------+
| 2975.00 |
+---------+
1 row in set (0.00 sec)单行多列（查询jones的信息）//select * from emp where ename=’jones‘；
mysql> select * from emp where ename='jones';
+-------+-------+---------+------+------------+---------+------+--------+
| empno | ename | job | mgr | hiredate | sal | comm | deptno |
+-------+-------+---------+------+------------+---------+------+--------+
| 7566 | JONES | MANAGER | 7839 | 1981-04-02 | 2975.00 | NULL | 20 |
+-------+-------+---------+------+------------+---------+------+--------+
1 row in set (0.00 sec)多行单列（查询某一列）select sal from emp；
mysql> select sal from emp;
+---------+
| sal |
+---------+
| 800.00 |
| 1600.00 |
| 1250.00 |
| 2975.00 |
| 1250.00 |
| 2850.00 |
| 2450.00 |
| 3000.00 |
| 5000.00 |
| 1500.00 |
| 1100.00 |
| 950.00 |
| 3000.00 |
| 1300.00 |
+---------+
14 rows in set (0.00 sec)- 工资高于30号部门所有人工资，其中30部门的所有人工资是子查询。//select * from emp where sal >(select sal from emp where deptno=30);
mysql> select * from emp where sal>all(select sal from emp where deptno=30); +-------+-------+-----------+------+------------+---------+------+--------+ | empno | ename | job | mgr | hiredate | sal | comm | deptno | +-------+-------+-----------+------+------------+---------+------+--------+ | 7566 | JONES | MANAGER | 7839 | 1981-04-02 | 2975.00 | NULL | 20 | | 7788 | SCOTT | ANALYST | 7566 | 1987-04-19 | 3000.00 | NULL | 20 | | 7839 | KING | PRESIDENT | NULL | 1981-11-17 | 5000.00 | NULL | 10 | | 7902 | FORD | ANALYST | 7566 | 1981-12-03 | 3000.00 | NULL | 20 | +-------+-------+-----------+------+------------+---------+------+--------+ 4 rows in set (0.00 sec)
(10)多表查询
多表查询有以下几种：
1.合并结果集：union、union all
作用：合并结果集就是把两个select语句的查询结果合并在一起；
union : 去除重复记录
union all ： 不去除重复记录
注意： 被合并的两个结果的列数和列类型必须相同。
/*!创建两个表book和person，并将这两个表合并*/
mysql> create table book(
-> id int,
-> name varchar(10)
-> );
Query OK, 0 rows affected (0.07 sec)
mysql> insert book(id,name)values(1,'红楼梦'),(2,'三国');
Query OK, 2 rows affected (0.03 sec)
Records: 2 Duplicates: 0 Warnings: 0
mysql> select * from book;
+------+--------+
| id | name |
+------+--------+
| 1 | 红楼梦 |
| 2 | 三国 |
+------+--------+
2 rows in set (0.00 sec)
mysql> create table person(
-> id int,
-> name varchar(10)
-> );
Query OK, 0 rows affected (0.14 sec)
mysql> insert person(id,name) values(2,'宋江'),(3,'贾宝玉');
Query OK, 2 rows affected (0.02 sec)
Records: 2 Duplicates: 0 Warnings: 0
mysql> select * from person;
+------+--------+
| id | name |
+------+--------+
| 2 | 宋江 |
| 3 | 贾宝玉 |
+------+--------+
2 rows in set (0.00 sec)
mysql> select *from book union select * from person;
+------+--------+
| id | name |
+------+--------+
| 1 | 红楼梦 |
| 2 | 三国 |
| 2 | 宋江 |
| 3 | 贾宝玉 |
+------+--------+
4 rows in set (0.00 sec)2.连接查询：
等值链接 // select * from 表名1 ，表名2 where 表名1.列名 操作符 表名2 .列名；
（a）内连接 【inter】 join on
select * from 表名1 inter join 表名2 on 连接条件 ；
（b）外连接 outer join on
select * from 表名1 outer join 表名2 on 连接条件 ；
（c）做外连接 left outer join 右外连接 right outer join
select * from 表名1 left outer join 表名2 on 连接条件 ；
（d）全连接（mysql不支持）
3.笛卡尔积
select * from 表名1，表名2；
4.外键（Foreign Key）约束 （常用“一对多”关联关系）
创建外键方式：在创建表的sql语句中使用：
constraint（约束名称）
foreign key（充当外键的列名）
references（主表名称）（与子表外键关联的列名）
mysql> create table student(
-> id int primary key,
-> name varchar(10),
-> sid int,
-> foreign key(sid) references school(id));
Query OK, 0 rows affected (0.14 sec)
mysql> insert school (id ,name)values(1,'清华'),(2,'千锋');
Query OK, 2 rows affected (0.04 sec)
Records: 2 Duplicates: 0 Warnings: 0
mysql> insert student(id,name,sid)values(1,'张三',2),(2,'李四',1),(3,'王五',2);
Query OK, 3 rows affected (0.08 sec)
Records: 3 Duplicates: 0 Warnings: 0
mysql> select * from school;
+----+------+
| id | name |
+----+------+
| 1 | 清华 |
| 2 | 千锋 |
+----+------+
2 rows in set (0.00 sec)
mysql> select * from student;
+----+------+------+
| id | name | sid |
+----+------+------+
| 1 | 张三 | 2 |
| 2 | 李四 | 1 |
| 3 | 王五 | 2 |
+----+------+------+
3 rows in set (0.00 sec)
mysql> select * from school,student;
+----+------+----+------+------+
| id | name | id | name | sid |
+----+------+----+------+------+
| 1 | 清华 | 1 | 张三 | 2 |
| 2 | 千锋 | 1 | 张三 | 2 |
| 1 | 清华 | 2 | 李四 | 1 |
| 2 | 千锋 | 2 | 李四 | 1 |
| 1 | 清华 | 3 | 王五 | 2 |
| 2 | 千锋 | 3 | 王五 | 2 |
+----+------+----+------+------+
6 rows in set (0.00 sec)
5.字符函数
concat（string1，string2）---连接两个字符串的内容
concat_WS（"连接符"，string1 ，string2）---使用连接符连接字串内容
upper（string）---将字串转成大写
lower（string）---将字串转成小写
left（string，num）----取一个字符串的前多少位
right（string，num）----取 一个字符串的后多少位
6.实用函数
>1 数值运算符和函数；
ceil（） 进一取整
div（） 整数除法
floor（） 舍一取整
mod（） 取余数或取模
power（a，b） 幂运算-----a^b
round() 四舍五入
truncate 数字截取--------------truncate（123.45，-2）---100
7.日期时间函数
now（） 当前日期和时间
curdate（） 当前日期
curtime（） 当前时间
date_add（） 日期变化
datediff（） 日期差值-----select datediff（now（），’2013-12-28'）;
date_format() 日期格式化
8.加密函数：
MD5（）----web加密----------不可逆（）
mysql> select md5('123');
+----------------------------------+
| md5('123') |
+----------------------------------+
| 202cb962ac59075b964b07152d234b70 |
+----------------------------------+
1 row in set (0.00 sec)
mysql> select md5('202cb962ac59075b964b07152d234b70');
+-----------------------------------------+
| md5('202cb962ac59075b964b07152d234b70') |
+-----------------------------------------+
| d9b1d7db4cd6e70935368a1efb10e377 |
+-----------------------------------------+
1 row in set (0.00 sec)password（）--------修改用户密码
总结：
查询语句书写顺序：
（select）--from--where--group by--having--order by--limit；
查询语句执行顺序：
from--where--group by--having--（select）--order by-- limit；