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MySQL多表查询

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多表查询

案列说明

笛卡尔积的理解

select id,department_name
from employees,departments;#错的

select id,department_id,department_name
from employees CROSS JOIN departments;#错的

每个员工和每个部门匹配了一遍(查出的条目数=id数*department数)

错误原因:缺少连接条件

笛卡尔积的解决

编写连接条件: 表1.列 = 表二.列(若多个表连接,至少要用n-1个连接条件)

select id,employees.name,department_name 
from employees,departments
WHERE employees.name = departments.name;

注:如果要显示的列在要查询的表中名字一样,则要表明,是出自哪个表, eg: employees.name

​ 建议在多表查询时,标明显示的是哪个表的信息 (优化)

优化:可以在FROM后使用表的别名,但是一旦使用别名,后续就一定要都用别名

多表查询的分类

等值连接和非等值连接

select t1.id,t1.name,t2.grade
from employees t1,departments t2
WHERE ti.salary BETWEEN t2.lowest_salary AND t2.highest_salary ;#非等值

自连接和非自连接

#显示员工(t1)和其管理者(t2)的基本信息
select t1.id,t1.name,t2.id,t2.name
from employees t1,employees t2#一个表看作两个表
WHERE t1.manage_id = t2.id ;#自连接

内连接和外连接

外连接的分类:左外连接(左表多,补右边),右外连接(右表多,补左边),满外连接

SQL92:使用(+)创建连接

内连接:见上

外连接:左表有数据不匹配,在右表加(+);反之,在左表加(+),但是MySQL不支持

WHERE t1.department_id = t2.department_id(+)#左连接

SQL99:使用JOIN...ON的方式

内连接

select t1.id,t1.name,t2.department_name,t3.environment
from employees t1 JOIN departments t2
ON t1.department_id = t2.department_id
JOIN locations t3#加入第二个人表
ON t2.department_location = t3.department_location;

外连接

使用OUTER JOIN...ON...

select t1.name,t2.department_name#左外连接
from employees t1 LEFT OUTER(可省略) JOIN departments t2
ON t1.department_id = t2.department_id;

UMION的使用

合并查询结果

SELECT colum... FROM table1
UNION (ALL)
SELECT colum... FROM table2

两个查询结果的并集,去重(效率低)

​ 两个查询结果的并集,不去重(效率高)

7种SQL JOINS的实现

中图(内连接):

select t1.name,t2.department_name
from employees t1 JOIN departments t2
ON t1.department_id = t2.department_id;

左上图(左外连接):

select t1.name,t2.department_name
from employees t1 LEFT JOIN departments t2
ON t1.department_id = t2.department_id;

右上图(右外连接):

select t1.name,t2.department_name
from employees t1 RIGHT JOIN departments t2
ON t1.department_id = t2.department_id;

左中图:

select t1.name,t2.department_name
from employees t1 LEFT JOIN departments t2
ON t1.department_id = t2.department_id
WHERE t2.department_id IS NULL;

右中图:

select t1.name,t2.department_name
from employees t1 RIGHT JOIN departments t2
ON t1.department_id = t2.department_id
WHERE t1.department_id IS NULL;

左下图(满外连接):

#方式一:左上图 UNION ALL 右中图
select t1.name,t2.department_name
from employees t1 LEFT JOIN departments t2
ON t1.department_id = t2.department_id
UNION ALL 
select t1.name,t2.department_name
from employees t1 RIGHT JOIN departments t2
ON t1.department_id = t2.department_id
WHERE t1.department_id IS NULL;

#方式二:左中图 UNION ALL 右上图
select t1.name,t2.department_name
from employees t1 LEFT JOIN departments t2
ON t1.department_id = t2.department_id
WHERE t2.department_id IS NULL
UNION ALL
select t1.name,t2.department_name
from employees t1 RIGHT JOIN departments t2
ON t1.department_id = t2.department_id;

右下图:

#左中图 UNION ALL 右中图
select t1.name,t2.department_name
from employees t1 LEFT JOIN departments t2
ON t1.department_id = t2.department_id
WHERE t2.department_id IS NULL
UNION ALL
select t1.name,t2.department_name
from employees t1 RIGHT JOIN departments t2
ON t1.department_id = t2.department_id
WHERE t1.department_id IS NULL;

SQL语法新特性

自然连接

使用关键字:NATURAL JOIN(不灵活),自动查询表中所有相同字段,然后进行等值连接

USING连接(不适用于自连接)

使用关键字:USING(同名字段),将表中相同名字的字段自动等值连接

select t1.name,t2.department_name
from employees t1 JOIN departments t2
ON t1.department_id = t2.department_id;
等价于
select t1.name,t2.department_name
from employees t1 JOIN departments t2
USING(department_id);

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