MySQL-连接查询

1. 连接查询

• –内连接查询

  JOIN|CROSS JOIN|INNER JOIN 通过ON连接条件，显示两个表中符合连接条件的记录
  

• –外连接查询

  左外连接 LEFT [OUTER] JOIN 显示左表的全部记录及右表符合连接条件的记录
  右外连接 RIGHT [OUTER] JOIN 显示右表的全部记录以及左表符合条件的记录
  

2. 内连接查询练习

• –查询用户表中的编号，用户名，省份表中的省份名字

SELECT id,username,proName FROM cms_user,provinces;		#错误
SELECT cms_user.id,username,proName FROM cms_user,provinces;	#指定表名

• –用户表cms_user中的proId对应省份表provinces中的id

SELECT cms_user.id,username,proName FROM cms_user,provinces WHERE cms_user.proId=provinces.id;

• –查询cms_user表中的id,username,email,sex，查询provinces表中的proName

SELECT a.id,a.username,a.email,a.sex,b.proName 
FROM cms_user AS a
INNER JOIN provices AS b
ON a.proId=b.id;

SELECT a.id,a.username,a.email,a.sex,b.proName 
FROM provinces AS b
CROSS JOIN cms_user AS a
ON a.proId=b.id;

• –INNER|CROSS可省略

SELECT a.id,a.username,a.email,a.sex,b.proName 
FROM provinces AS b
JOIN cms_user AS a
ON a.proId=b.id;

• –查询cms_user表中的 id,username,sex，查询provinces表中的 proName，条件是cms_user中性别为男的用户

SELECT a.id,a.username,a.sex,b.proName
FROM cms_user AS a
JOIN provinces AS b
ON a.proId=b.id
WHERE u.sex='男'；

• –再根据proName分组

SELECT a.id,a.username,a.sex,b.proName,COUNT(*) AS totalUsers
FROM cms_user AS a
JOIN provinces AS b
ON a.proId=b.id
WHERE u.sex='男'
GROUP BY b.proName;

• –再对分组结果进行二次筛选，选出组中人数大于等于1的记录

SELECT a.id,a.username,a.sex,b.proName,COUNT(*) AS totalUsers,GROUP_CONCAT(username)
FROM cms_user AS a
JOIN provinces AS b
ON a.proId=b.id
WHERE u.sex='男'
GROUP BY b.proName
HAVING COUNT(*) >=1;

• –再按照id升序排序

SELECT a.id,a.username,a.sex,b.proName,COUNT(*) AS totalUsers,GROUP_CONCAT(username)
FROM cms_user AS a
JOIN provinces AS b
ON a.proId=b.id
WHERE u.sex='男'
GROUP BY b.proName
HAVING COUNT(*) >=1
ORDER BY a.id ASC;

• –查询cms_news中的id,title，查询cms_cate中的cateName

SELECT a.id,a.title,b.cateName 
FROM cms_news AS a
JOIN cms_cate AS b
ON a.cId=b.id;

• –查询cms_news中的id,title，查询cms_admin中的username,role

SELECT a.id,a.title,b.username,b.role 
FROM cms_news AS a
JOIN cms_admin AS b
ON a.aId=b.id;

• –三表联查；查询cms_news中的id,title，查询cms_cate中的cateName，查询cms_admin中的username,role

SELECT n.id,n.title,c.cateName,a.username,a.role
FROM cms_news AS n
JOIN cms_cate AS c
ON n.cId=c.id
JOIN cms_admin AS a
ON n.aId=a.id;

3. 外连接查询练习

• –左外连接（以左表为主表）

SELECT a.id,a.username,a.email,a.sex,b.proName 
FROM cms_user AS a
LEFT JOIN provices AS b
ON a.proId=b.id;

• –右外连接（以右表为主表）

SELECT a.id,a.username,a.email,a.sex,b.proName 
FROM provices AS b
RIGHT JOIN cms_user AS a
ON a.proId=b.id;