MySQL知识点总结(2)
迪丽瓦拉
2024-05-30 16:38:12
0次
目录
1、关于查询结果集的去重？
2、连接查询
3、子查询
4、union （可以将查询结果集相加）
5、limit (重点中的重点，以后分页查询全靠它了。)
6、创建表
7、insert语句插入数据
8、表的复制
9、将查询结果插入到一张表中
10、修改数据：update
11、删除数据
12、增删改查
13、约束(Constraint)
1、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;//联合去重
+--------+-----------+
| 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 |
+---------------------+
2、连接查询
2.1、什么是连接查询？在实际开发中，大部分的情况下都不是从单表中查询数据，一般都是多张表联合查询取出最终的结果。在实际开发中，一般一个业务都会对应多张表，比如：学生和班级，起码两张表。stuno stuname classno classname-----------------------------------------------------------------------------------1 zs 1 北京大兴区亦庄经济技术开发区第二中学高三1班2 ls 1 北京大兴区亦庄经济技术开发区第二中学高三1班...学生和班级信息存储到一张表中，结果就像上面一样，数据会存在大量的重复，导致数据的冗余。2.2、连接查询的分类？根据语法出现的年代来划分的话，包括：SQL92（一些老的DBA可能还在使用这种语法。DBA：DataBase Administrator，数据库管理员）SQL99（比较新的语法）根据表的连接方式来划分，包括：内连接：等值连接非等值连接自连接外连接：左外连接（左连接）右外连接（右连接）全连接（这个不讲，很少用！）2.3、在表的连接查询方面有一种现象被称为：笛卡尔积现象。（笛卡尔乘积现象）案例：找出每一个员工的部门名称，要求显示员工名和部门名。
EMP表
+--------+--------+
| ename | deptno |
+--------+--------+
| SMITH | 20 |
| ALLEN | 30 |
| WARD | 30 |
| JONES | 20 |
| MARTIN | 30 |
| BLAKE | 30 |
| CLARK | 10 |
| SCOTT | 20 |
| KING | 10 |
| TURNER | 30 |
| ADAMS | 20 |
| JAMES | 30 |
| FORD | 20 |
| MILLER | 10 |
+--------+--------+
DEPT表
+--------+------------+----------+
| DEPTNO | DNAME | LOC |
+--------+------------+----------+
| 10 | ACCOUNTING | NEW YORK |
| 20 | RESEARCH | DALLAS |
| 30 | SALES | CHICAGO |
| 40 | OPERATIONS | BOSTON |
+--------+------------+----------+select ename,dname from emp,dept;
+--------+------------+
| ename | dname |
+--------+------------+
| SMITH | ACCOUNTING |
| SMITH | RESEARCH |
| SMITH | SALES |
| SMITH | OPERATIONS |
| ALLEN | ACCOUNTING |
| ALLEN | RESEARCH |
| ALLEN | SALES |
| ALLEN | OPERATIONS |
............
56 rows in set (0.00 sec)笛卡尔积现象：当两张表进行连接查询的时候，没有任何条件进行限制，最终的查询结果条数是两张表记录条数的乘积。关于表的别名：select e.ename,d.dname from emp e,dept d;表的别名有什么好处？第一：执行效率高。第二：可读性好。2.4、怎么避免笛卡尔积现象？当然是加条件进行过滤。
思考：避免了笛卡尔积现象，会减少记录的匹配次数吗？不会，次数还是56次。只不过显示的是有效记录。案例：找出每一个员工的部门名称，要求显示员工名和部门名。select e.ename,d.dnamefromemp e , dept dwheree.deptno = d.deptno; //SQL92，以后不用。+--------+------------+| ename | dname |+--------+------------+| CLARK | ACCOUNTING || KING | ACCOUNTING || MILLER | ACCOUNTING || SMITH | RESEARCH || JONES | RESEARCH || SCOTT | RESEARCH || ADAMS | RESEARCH || FORD | RESEARCH || ALLEN | SALES || WARD | SALES || MARTIN | SALES || BLAKE | SALES || TURNER | SALES || JAMES | SALES |+--------+------------+2.5、内连接之等值连接：最大特点是：条件是等量关系。案例：查询每个员工的部门名称，要求显示员工名和部门名。SQL92:（太老，不用了）select e.ename,d.dnamefromemp e, dept dwheree.deptno = d.deptno;SQL99：（常用的）select e.ename,d.dnamefromemp ejoindept done.deptno = d.deptno;// inner可以省略的，带着inner目的是可读性好一些。select e.ename,d.dnamefromemp einner joindept done.deptno = d.deptno;语法：...AjoinBon连接条件where...SQL99语法结构更清晰一些：表的连接条件和后来的where条件分离了。+--------+------------+| ename | dname |+--------+------------+| CLARK | ACCOUNTING || KING | ACCOUNTING || MILLER | ACCOUNTING || SMITH | RESEARCH || JONES | RESEARCH || SCOTT | RESEARCH || ADAMS | RESEARCH || FORD | RESEARCH || ALLEN | SALES || WARD | SALES || MARTIN | SALES || BLAKE | SALES || TURNER | SALES || JAMES | SALES |+--------+------------+2.6、内连接之非等值连接：最大的特点是：连接条件中的关系是非等量关系。案例：找出每个员工的工资等级，要求显示员工名、工资、工资等级。
mysql> select ename,sal from emp; e
+--------+---------+
| ename | sal |
+--------+---------+
| SMITH | 800.00 |
| ALLEN | 1600.00 |
| WARD | 1250.00 |
| JONES | 2975.00 |
| MARTIN | 1250.00 |
| BLAKE | 2850.00 |
| CLARK | 2450.00 |
| SCOTT | 3000.00 |
| KING | 5000.00 |
| TURNER | 1500.00 |
| ADAMS | 1100.00 |
| JAMES | 950.00 |
| FORD | 3000.00 |
| MILLER | 1300.00 |
+--------+---------+mysql> select * from salgrade; s
+-------+-------+-------+
| GRADE | LOSAL | HISAL |
+-------+-------+-------+
| 1 | 700 | 1200 |
| 2 | 1201 | 1400 |
| 3 | 1401 | 2000 |
| 4 | 2001 | 3000 |
| 5 | 3001 | 9999 |
+-------+-------+-------+select e.ename,e.sal,s.grade
fromemp e
joinsalgrade s
one.sal between s.losal and s.hisal;// inner可以省略
select e.ename,e.sal,s.grade
fromemp e
inner joinsalgrade s
one.sal between s.losal and s.hisal;+--------+---------+-------+
| ename | sal | grade |
+--------+---------+-------+
| SMITH | 800.00 | 1 |
| ALLEN | 1600.00 | 3 |
| WARD | 1250.00 | 2 |
| JONES | 2975.00 | 4 |
| MARTIN | 1250.00 | 2 |
| BLAKE | 2850.00 | 4 |
| CLARK | 2450.00 | 4 |
| SCOTT | 3000.00 | 4 |
| KING | 5000.00 | 5 |
| TURNER | 1500.00 | 3 |
| ADAMS | 1100.00 | 1 |
| JAMES | 950.00 | 1 |
| FORD | 3000.00 | 4 |
| MILLER | 1300.00 | 2 |
+--------+---------+-------+2.7、自连接：最大的特点是：一张表看做两张表。自己连接自己。案例：找出每个员工的上级领导，要求显示员工名和对应的领导名。
mysql> select empno,ename,mgr from emp;
emp a 员工表
+-------+--------+------+
| empno | ename | mgr |
+-------+--------+------+
| 7369 | SMITH | 7902 |
| 7499 | ALLEN | 7698 |
| 7521 | WARD | 7698 |
| 7566 | JONES | 7839 |
| 7654 | MARTIN | 7698 |
| 7698 | BLAKE | 7839 |
| 7782 | CLARK | 7839 |
| 7788 | SCOTT | 7566 |
| 7839 | KING | NULL |
| 7844 | TURNER | 7698 |
| 7876 | ADAMS | 7788 |
| 7900 | JAMES | 7698 |
| 7902 | FORD | 7566 |
| 7934 | MILLER | 7782 |
+-------+--------+------+
emp b 领导表
+-------+--------+
| empno | ename |
+-------+--------+
| 7566 | JONES |
| 7698 | BLAKE |
| 7782 | CLARK |
| 7788 | SCOTT |
| 7839 | KING |
| 7902 | FORD |
+-------+--------+员工的领导编号 = 领导的员工编号select a.ename as '员工名',b.ename as '领导名'
fromemp a
inner joinemp b
ona.mgr = b.empno;+--------+--------+
| 员工名 | 领导名 |
+--------+--------+
| SMITH | FORD |
| ALLEN | BLAKE |
| WARD | BLAKE |
| JONES | KING |
| MARTIN | BLAKE |
| BLAKE | KING |
| CLARK | KING |
| SCOTT | JONES |
| TURNER | BLAKE |
| ADAMS | SCOTT |
| JAMES | BLAKE |
| FORD | JONES |
| MILLER | CLARK |
+--------+--------+2.8、外连接？什么是外连接，和内连接有什么区别？内连接：假设A和B表进行连接，使用内连接的话，凡是A表和B表能够匹配上的记录查询出来，这就是内连接。AB两张表没有主副之分，两张表是平等的。外连接：假设A和B表进行连接，使用外连接的话，AB两张表中有一张表是主表，一张表是副表，主要查询主表中的数据，捎带着查询副表，当副表中的数据没有和主表中的数据匹配上，副表自动模拟出NULL与之匹配。外连接的分类？左外连接（左连接）：表示左边的这张表是主表。右外连接（右连接）：表示右边的这张表是主表。左连接有右连接的写法，右连接也会有对应的左连接的写法。案例：找出每个员工的上级领导？（所有员工必须全部查询出来。）emp a 员工表
+-------+--------+------+
| empno | ename | mgr |
+-------+--------+------+
| 7369 | SMITH | 7902 |
| 7499 | ALLEN | 7698 |
| 7521 | WARD | 7698 |
| 7566 | JONES | 7839 |
| 7654 | MARTIN | 7698 |
| 7698 | BLAKE | 7839 |
| 7782 | CLARK | 7839 |
| 7788 | SCOTT | 7566 |
| 7839 | KING | NULL |
| 7844 | TURNER | 7698 |
| 7876 | ADAMS | 7788 |
| 7900 | JAMES | 7698 |
| 7902 | FORD | 7566 |
| 7934 | MILLER | 7782 |
+-------+--------+------+
emp b 领导表
+-------+--------+
| empno | ename |
+-------+--------+
| 7566 | JONES |
| 7698 | BLAKE |
| 7782 | CLARK |
| 7788 | SCOTT |
| 7839 | KING |
| 7902 | FORD |
+-------+--------+内连接：
select a.ename '员工', b.ename '领导'
fromemp a
joinemp b
ona.mgr = b.empno;外连接：（左外连接/左连接）
select a.ename '员工', b.ename '领导'
fromemp a
left joinemp b
ona.mgr = b.empno;// outer是可以省略的。
select a.ename '员工', b.ename '领导'
fromemp a
left outer joinemp b
ona.mgr = b.empno;外连接：（右外连接/右连接）
select a.ename '员工', b.ename '领导'
fromemp b
right joinemp a
ona.mgr = b.empno;// outer可以省略。
select a.ename '员工', b.ename '领导'
fromemp b
right outer joinemp a
ona.mgr = b.empno;+--------+-------+
| 员工 | 领导 |
+--------+-------+
| SMITH | FORD |
| ALLEN | BLAKE |
| WARD | BLAKE |
| JONES | KING |
| MARTIN | BLAKE |
| BLAKE | KING |
| CLARK | KING |
| SCOTT | JONES |
| KING | NULL |
| TURNER | BLAKE |
| ADAMS | SCOTT |
| JAMES | BLAKE |
| FORD | JONES |
| MILLER | CLARK |
+--------+-------+外连接最重要的特点是：主表的数据无条件的全部查询出来。案例：找出哪个部门没有员工？
EMP表
+-------+--------+-----------+------+------------+---------+---------+--------+
| 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 |
+-------+--------+-----------+------+------------+---------+---------+--------+
DEPT
+--------+------------+----------+
| DEPTNO | DNAME | LOC |
+--------+------------+----------+
| 10 | ACCOUNTING | NEW YORK |
| 20 | RESEARCH | DALLAS |
| 30 | SALES | CHICAGO |
| 40 | OPERATIONS | BOSTON |
+--------+------------+----------+select d.*
fromemp e
right joindept d
one.deptno = d.deptno
wheree.empno is null;+--------+------------+--------+
| DEPTNO | DNAME | LOC |
+--------+------------+--------+
| 40 | OPERATIONS | BOSTON |
+--------+------------+--------+2.9、三张表怎么连接查询？
案例：找出每一个员工的部门名称以及工资等级。
EMP e
+-------+--------+---------+--------+
| empno | ename | sal | deptno |
+-------+--------+---------+--------+
| 7369 | SMITH | 800.00 | 20 |
| 7499 | ALLEN | 1600.00 | 30 |
| 7521 | WARD | 1250.00 | 30 |
| 7566 | JONES | 2975.00 | 20 |
| 7654 | MARTIN | 1250.00 | 30 |
| 7698 | BLAKE | 2850.00 | 30 |
| 7782 | CLARK | 2450.00 | 10 |
| 7788 | SCOTT | 3000.00 | 20 |
| 7839 | KING | 5000.00 | 10 |
| 7844 | TURNER | 1500.00 | 30 |
| 7876 | ADAMS | 1100.00 | 20 |
| 7900 | JAMES | 950.00 | 30 |
| 7902 | FORD | 3000.00 | 20 |
| 7934 | MILLER | 1300.00 | 10 |
+-------+--------+---------+--------+
DEPT d
+--------+------------+----------+
| DEPTNO | DNAME | LOC |
+--------+------------+----------+
| 10 | ACCOUNTING | NEW YORK |
| 20 | RESEARCH | DALLAS |
| 30 | SALES | CHICAGO |
| 40 | OPERATIONS | BOSTON |
+--------+------------+----------+
SALGRADE s
+-------+-------+-------+
| GRADE | LOSAL | HISAL |
+-------+-------+-------+
| 1 | 700 | 1200 |
| 2 | 1201 | 1400 |
| 3 | 1401 | 2000 |
| 4 | 2001 | 3000 |
| 5 | 3001 | 9999 |
+-------+-------+-------+注意，解释一下：....AjoinBjoinCon...表示：A表和B表先进行表连接，连接之后A表继续和C表进行连接。select e.ename,d.dname,s.gradefromemp ejoindept done.deptno = d.deptnojoinsalgrade sone.sal between s.losal and s.hisal;+--------+------------+-------+| ename | dname | grade |+--------+------------+-------+| SMITH | RESEARCH | 1 || ALLEN | SALES | 3 || WARD | SALES | 2 || JONES | RESEARCH | 4 || MARTIN | SALES | 2 || BLAKE | SALES | 4 || CLARK | ACCOUNTING | 4 || SCOTT | RESEARCH | 4 || KING | ACCOUNTING | 5 || TURNER | SALES | 3 || ADAMS | RESEARCH | 1 || JAMES | SALES | 1 || FORD | RESEARCH | 4 || MILLER | ACCOUNTING | 2 |+--------+------------+-------+案例：找出每一个员工的部门名称、工资等级、以及上级领导。select e.ename '员工',d.dname,s.grade,e1.ename '领导'fromemp ejoindept done.deptno = d.deptnojoinsalgrade sone.sal between s.losal and s.hisalleft joinemp e1one.mgr = e1.empno;+--------+------------+-------+-------+| 员工 | dname | grade | 领导 |+--------+------------+-------+-------+| SMITH | RESEARCH | 1 | FORD || ALLEN | SALES | 3 | BLAKE || WARD | SALES | 2 | BLAKE || JONES | RESEARCH | 4 | KING || MARTIN | SALES | 2 | BLAKE || BLAKE | SALES | 4 | KING || CLARK | ACCOUNTING | 4 | KING || SCOTT | RESEARCH | 4 | JONES || KING | ACCOUNTING | 5 | NULL || TURNER | SALES | 3 | BLAKE || ADAMS | RESEARCH | 1 | SCOTT || JAMES | SALES | 1 | BLAKE || FORD | RESEARCH | 4 | JONES || MILLER | ACCOUNTING | 2 | CLARK |+--------+------------+-------+-------+
3、子查询
3.1、什么是子查询？子查询都可以出现在哪里？select语句当中嵌套select语句，被嵌套的select语句是子查询。子查询可以出现在哪里？select..(select).from..(select).where..(select).3.2、where子句中使用子查询案例：找出高于平均薪资的员工信息。
select * from emp where sal > avg(sal); //错误的写法，where后面不能直接使用分组函数。第一步：找出平均薪资select avg(sal) from emp;+-------------+| avg(sal) |+-------------+| 2073.214286 |+-------------+
第二步：where过滤select * from emp where sal > 2073.214286;+-------+-------+-----------+------+------------+---------+------+--------+| EMPNO | ENAME | JOB | MGR | HIREDATE | SAL | COMM | DEPTNO |+-------+-------+-----------+------+------------+---------+------+--------+| 7566 | JONES | MANAGER | 7839 | 1981-04-02 | 2975.00 | NULL | 20 || 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 || 7902 | FORD | ANALYST | 7566 | 1981-12-03 | 3000.00 | NULL | 20 |+-------+-------+-----------+------+------------+---------+------+--------+
第一步和第二步合并：select * from emp where sal > (select avg(sal) from emp);3.3、from后面嵌套子查询案例：找出每个部门平均薪水的等级。
第一步：找出每个部门平均薪水（按照部门编号分组，求sal的平均值）
select deptno,avg(sal) as avgsal from emp group by deptno;
+--------+-------------+
| deptno | avgsal |
+--------+-------------+
| 10 | 2916.666667 |
| 20 | 2175.000000 |
| 30 | 1566.666667 |
+--------+-------------+
第二步：将以上的查询结果当做临时表t，让t表和salgrade s表连接，条件是：t.avgsal between s.losal and s.hisal
select t.*,s.grade
from(select deptno,avg(sal) as avgsal from emp group by deptno) t
joinsalgrade s
ont.avgsal between s.losal and s.hisal;+--------+-------------+-------+
| deptno | avgsal | grade |
+--------+-------------+-------+
| 30 | 1566.666667 | 3 |
| 10 | 2916.666667 | 4 |
| 20 | 2175.000000 | 4 |
+--------+-------------+-------+案例：找出每个部门平均的薪水等级。
第一步：找出每个员工的薪水等级。
select e.ename,e.sal,e.deptno,s.grade from emp e join salgrade s on e.sal between s.losal and s.hisal;
+--------+---------+--------+-------+
| ename | sal | deptno | grade |
+--------+---------+--------+-------+
| SMITH | 800.00 | 20 | 1 |
| ALLEN | 1600.00 | 30 | 3 |
| WARD | 1250.00 | 30 | 2 |
| JONES | 2975.00 | 20 | 4 |
| MARTIN | 1250.00 | 30 | 2 |
| BLAKE | 2850.00 | 30 | 4 |
| CLARK | 2450.00 | 10 | 4 |
| SCOTT | 3000.00 | 20 | 4 |
| KING | 5000.00 | 10 | 5 |
| TURNER | 1500.00 | 30 | 3 |
| ADAMS | 1100.00 | 20 | 1 |
| JAMES | 950.00 | 30 | 1 |
| FORD | 3000.00 | 20 | 4 |
| MILLER | 1300.00 | 10 | 2 |
+--------+---------+--------+-------+
第二步：基于以上结果，继续按照deptno分组，求grade平均值。
select e.deptno,avg(s.grade)
from emp e
join salgrade s
on e.sal between s.losal and s.hisal
group bye.deptno;+--------+--------------+
| deptno | avg(s.grade) |
+--------+--------------+
| 10 | 3.6667 |
| 20 | 2.8000 |
| 30 | 2.5000 |
+--------+--------------+3.4、在select后面嵌套子查询。
案例：找出每个员工所在的部门名称，要求显示员工名和部门名。select e.ename,d.dname
fromemp e
joindept d
one.deptno = d.deptno;select e.ename,(select d.dname from dept d where e.deptno = d.deptno) as dname
from emp e;+--------+------------+
| ename | dname |
+--------+------------+
| SMITH | RESEARCH |
| ALLEN | SALES |
| WARD | SALES |
| JONES | RESEARCH |
| MARTIN | SALES |
| BLAKE | SALES |
| CLARK | ACCOUNTING |
| SCOTT | RESEARCH |
| KING | ACCOUNTING |
| TURNER | SALES |
| ADAMS | RESEARCH |
| JAMES | SALES |
| FORD | RESEARCH |
| MILLER | ACCOUNTING |
+--------+------------+
4、union （可以将查询结果集相加）
案例：找出工作岗位是SALESMAN和MANAGER的员工？
第一种：select ename,job from emp where job = 'MANAGER' or job = 'SALESMAN';
第二种：select ename,job from emp where job in('MANAGER','SALESMAN');
+--------+----------+
| ename | job |
+--------+----------+
| ALLEN | SALESMAN |
| WARD | SALESMAN |
| JONES | MANAGER |
| MARTIN | SALESMAN |
| BLAKE | MANAGER |
| CLARK | MANAGER |
| TURNER | SALESMAN |
+--------+----------+
第三种：union
select ename,job from emp where job = 'MANAGER'
union
select ename,job from emp where job = 'SALESMAN';
+--------+----------+
| ename | job |
+--------+----------+
| JONES | MANAGER |
| BLAKE | MANAGER |
| CLARK | MANAGER |
| ALLEN | SALESMAN |
| WARD | SALESMAN |
| MARTIN | SALESMAN |
| TURNER | SALESMAN |
+--------+----------+两张不相干的表中的数据拼接在一起显示？
select ename from emp
union
select dname from dept;+------------+
| ename |
+------------+
| SMITH |
| ALLEN |
| WARD |
| JONES |
| MARTIN |
| BLAKE |
| CLARK |
| SCOTT |
| KING |
| TURNER |
| ADAMS |
| JAMES |
| FORD |
| MILLER |
| ACCOUNTING |
| RESEARCH |
| SALES |
| OPERATIONS |
+------------+mysql> select ename,sal from emp-> union-> select dname from dept;
ERROR 1222 (21000): The used SELECT statements have a different number of columns
5、limit (重点中的重点，以后分页查询全靠它了。)
5.1、limit是mysql特有的，其他数据库中没有，不通用。（Oracle中有一个相同的机制，叫做rownum）5.2、limit取结果集中的部分数据，这时它的作用。5.3、语法机制：limit startIndex, lengthstartIndex表示起始位置，从0开始，0表示第一条数据。length表示取几个案例：取出工资前5名的员工（思路：降序取前5个）select ename,sal from emp order by sal desc;取前5个：select ename,sal from emp order by sal desc limit 0, 5;select ename,sal from emp order by sal desc limit 5;5.4、limit是sql语句最后执行的一个环节：select 5...from 1... where 2... group by 3...having 4...order by 6...limit 7...;5.5、案例：找出工资排名在第4到第9名的员工？select ename,sal from emp order by sal desc limit 3,6;+--------+---------+| ename | sal |+--------+---------+| JONES | 2975.00 || BLAKE | 2850.00 || CLARK | 2450.00 || ALLEN | 1600.00 || TURNER | 1500.00 || MILLER | 1300.00 |+--------+---------+5.6、通用的标准分页sql？每页显示3条记录：
第1页：0, 3
第2页：3, 3
第3页：6, 3
第4页：9, 3
第5页：12, 3每页显示pageSize条记录：
第pageNo页：(pageNo - 1) * pageSize, pageSizepageSize是什么？是每页显示多少条记录
pageNo是什么？显示第几页java代码{int pageNo = 2; // 页码是2int pageSize = 10; // 每页显示10条limit (pageNo - 1) * pageSize, pageSize
}
6、创建表
建表语句的语法格式：create table 表名(字段名1 数据类型,字段名2 数据类型,字段名3 数据类型,....);关于MySQL当中字段的数据类型？以下只说常见的int 整数型(java中的int)bigint 长整型(java中的long)float 浮点型(java中的float double)char 定长字符串(String)varchar 可变长字符串(StringBuffer/StringBuilder)date 日期类型 （对应Java中的java.sql.Date类型）BLOB 二进制大对象（存储图片、视频等流媒体信息） Binary Large OBject （对应java中的Object）CLOB 字符大对象（存储较大文本，比如，可以存储4G的字符串。） Character Large OBject（对应java中的Object）......char和varchar怎么选择？在实际的开发中，当某个字段中的数据长度不发生改变的时候，是定长的，例如：性别、生日等都是采用char。当一个字段的数据长度不确定，例如：简介、姓名等都是采用varchar。BLOB和CLOB类型的使用？电影表: t_movieid(int) name(varchar) playtime(date/char) haibao(BLOB) history(CLOB)----------------------------------------------------------------------------------------1 蜘蛛侠 23表名在数据库当中一般建议以：t_或者tbl_开始。创建学生表：学生信息包括：学号、姓名、性别、班级编号、生日学号：bigint姓名：varchar性别：char班级编号：int生日：charcreate table t_student(no bigint,name varchar(255),sex char(1),classno varchar(255),birth char(10));
7、insert语句插入数据
语法格式：insert into 表名(字段名1,字段名2,字段名3,....) values(值1,值2,值3,....)要求：字段的数量和值的数量相同，并且数据类型要对应相同。insert into t_student(no,name,sex,classno,birth) values(1,'zhangsan','1','gaosan1ban');ERROR 1136 (21S01): Column count doesn't match value count at row 1insert into t_student(no,name,sex,classno,birth) values(1,'zhangsan','1','gaosan1ban', '1950-10-12');mysql> select * from t_student;+------+----------+------+------------+------------+| no | name | sex | classno | birth |+------+----------+------+------------+------------+| 1 | zhangsan | 1 | gaosan1ban | 1950-10-12 |+------+----------+------+------------+------------+insert into t_student(name,sex,classno,birth,no) values('lisi','1','gaosan1ban', '1950-10-12',2);mysql> select * from t_student;+------+----------+------+------------+------------+| no | name | sex | classno | birth |+------+----------+------+------------+------------+| 1 | zhangsan | 1 | gaosan1ban | 1950-10-12 || 2 | lisi | 1 | gaosan1ban | 1950-10-12 |+------+----------+------+------------+------------+insert into t_student(name) values('wangwu'); // 除name字段之外，剩下的所有字段自动插入NULL。mysql> select * from t_student;+------+----------+------+------------+------------+| no | name | sex | classno | birth |+------+----------+------+------------+------------+| 1 | zhangsan | 1 | gaosan1ban | 1950-10-12 || 2 | lisi | 1 | gaosan1ban | 1950-10-12 || NULL | wangwu | NULL | NULL | NULL |+------+----------+------+------------+------------+insert into t_student(no) values(3); mysql> select * from t_student;+------+----------+------+------------+------------+| no | name | sex | classno | birth |+------+----------+------+------------+------------+| 1 | zhangsan | 1 | gaosan1ban | 1950-10-12 || 2 | lisi | 1 | gaosan1ban | 1950-10-12 || NULL | wangwu | NULL | NULL | NULL || 3 | NULL | NULL | NULL | NULL |+------+----------+------+------------+------------+drop table if exists t_student; // 当这个表存在的话删除。create table t_student(no bigint,name varchar(255),sex char(1) default 1,classno varchar(255),birth char(10));insert into t_student(name) values('zhangsan');mysql> select * from t_student;+------+----------+------+---------+-------+| no | name | sex | classno | birth |+------+----------+------+---------+-------+| NULL | zhangsan | 1 | NULL | NULL |+------+----------+------+---------+-------+需要注意的地方：当一条insert语句执行成功之后，表格当中必然会多一行记录。即使多的这一行记录当中某些字段是NULL，后期也没有办法在执行insert语句插入数据了，只能使用update进行更新。// 字段可以省略不写，但是后面的value对数量和顺序都有要求。insert into t_student values(1,'jack','0','gaosan2ban','1986-10-23');mysql> select * from t_student;+------+----------+------+------------+------------+| no | name | sex | classno | birth |+------+----------+------+------------+------------+| NULL | zhangsan | 1 | NULL | NULL || 1 | jack | 0 | gaosan2ban | 1986-10-23 |+------+----------+------+------------+------------+insert into t_student values(1,'jack','0','gaosan2ban');ERROR 1136 (21S01): Column count doesn't match value count at row 1// 一次插入多行数据insert into t_student(no,name,sex,classno,birth) values(3,'rose','1','gaosi2ban','1952-12-14'),(4,'laotie','1','gaosi2ban','1955-12-14');mysql> select * from t_student;+------+----------+------+------------+------------+| no | name | sex | classno | birth |+------+----------+------+------------+------------+| NULL | zhangsan | 1 | NULL | NULL || 1 | jack | 0 | gaosan2ban | 1986-10-23 || 3 | rose | 1 | gaosi2ban | 1952-12-14 || 4 | laotie | 1 | gaosi2ban | 1955-12-14 |+------+----------+------+------------+------------+
8、表的复制
语法：create table 表名 as select语句;create table emp1 as select * from emp;create table emp2 as select empno,ename from emp;将查询结果当做表创建出来。9、将查询结果插入到一张表中
mysql> insert into dept1 select * from dept;mysql> select * from dept1;+--------+------------+----------+| DEPTNO | DNAME | LOC |+--------+------------+----------+| 10 | ACCOUNTING | NEW YORK || 20 | RESEARCH | DALLAS || 30 | SALES | CHICAGO || 40 | OPERATIONS | BOSTON || 10 | ACCOUNTING | NEW YORK || 20 | RESEARCH | DALLAS || 30 | SALES | CHICAGO || 40 | OPERATIONS | BOSTON |+--------+------------+----------+
10、修改数据：update
语法格式：update 表名 set 字段名1=值1,字段名2=值2... where 条件;注意：没有条件整张表数据全部更新。案例：将部门10的LOC修改为SHANGHAI，将部门名称修改为RENSHIBUupdate dept1 set loc = 'SHANGHAI', dname = 'RENSHIBU' where deptno = 10;mysql> select * from dept1;+--------+------------+----------+| DEPTNO | DNAME | LOC |+--------+------------+----------+| 10 | RENSHIBU | SHANGHAI || 20 | RESEARCH | DALLAS || 30 | SALES | CHICAGO || 40 | OPERATIONS | BOSTON || 10 | RENSHIBU | SHANGHAI || 20 | RESEARCH | DALLAS || 30 | SALES | CHICAGO || 40 | OPERATIONS | BOSTON |+--------+------------+----------+更新所有记录update dept1 set loc = 'x', dname = 'y';mysql> select * from dept1;+--------+-------+------+| DEPTNO | DNAME | LOC |+--------+-------+------+| 10 | y | x || 20 | y | x || 30 | y | x || 40 | y | x || 10 | y | x || 20 | y | x || 30 | y | x || 40 | y | x |+--------+-------+------+11、删除数据
语法格式：delete from 表名 where 条件;注意：没有条件全部删除。删除10部门数据？delete from dept1 where deptno = 10;删除所有记录？delete from dept1;怎么删除大表中的数据？（重点）truncate table 表名; // 表被截断，不可回滚。永久丢失。删除表？drop table 表名; // 这个通用。drop table if exists 表名; // oracle不支持这种写法。
12、增删改查
对于表结构的修改，这里不讲了，大家使用工具完成即可，因为在实际开发中表一旦
设计好之后，对表结构的修改是很少的，修改表结构就是对之前的设计进行了否定，即使
需要修改表结构，我们也可以直接使用工具操作。修改表结构的语句不会出现在Java代码当中。
出现在java代码当中的sql包括：insert delete update select（这些都是表中的数据操作。）增删改查有一个术语：CRUD操作
Create（增） Retrieve（检索） Update（修改） Delete（删除）13、约束(Constraint)
13.1、什么是约束？常见的约束有哪些呢？在创建表的时候，可以给表的字段添加相应的约束，添加约束的目的是为了保证表中数据的合法性、有效性、完整性。常见的约束有哪些呢？非空约束(not null)：约束的字段不能为NULL唯一约束(unique)：约束的字段不能重复主键约束(primary key)：约束的字段既不能为NULL，也不能重复（简称PK）外键约束(foreign key)：...（简称FK）检查约束(check)：注意Oracle数据库有check约束，但是mysql没有，目前mysql不支持该约束。13.2、非空约束 not nulldrop table if exists t_user;create table t_user(id int,username varchar(255) not null,password varchar(255));insert into t_user(id,password) values(1,'123');ERROR 1364 (HY000): Field 'username' doesn't have a default valueinsert into t_user(id,username,password) values(1,'lisi','123');
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