| 操作 | 描述 |
| inner join | 如果表中至少有一个匹配，就返回行 |
| left join | 会从左表中返回所有的值，即使右表中没有匹配 |
| right join? | 会从右表中返回所有的值，即使左表中没有匹配 |
语法：join? ?（连接的表）on(判断的条件)? ?
? ? ? ? ? -- where 等值查询
? ? ? ? ? --on? ?连接查询????????
代码案例：
-- 联表查询
-- inner join
SELECT s.studentno,studentname,subjectno,studentresult
FROM student AS s
INNER JOIN result AS r
WHERE s.studentno = r.studentno
-- right join
SELECT s.studentno,studentname,subjectno,studentresult
FROM student AS s
RIGHT JOIN result AS r
ON s.studentno = r.studentno
-- left join
SELECT s.studentno,studentname,subjectno,studentresult
FROM student AS s
LEFT JOIN result AS r
ON s.studentno = r.studentno多表查询：
1.我要查询哪些数据select……
2.从哪几个表中查，from 表? ?xxx? ?join? ? 连接的表? ? ? on? ? ? 交叉条件
3.假设存在一种多张表查询，慢慢来先查询两张表，然后再慢慢增加表
代码案例：
-- 查询缺考的同学
SELECT s.studentno,studentname,subjectno,studentresult
FROM student AS s
LEFT JOIN result AS r
ON s.studentno = r.studentno
WHERE `studentresult`IS NULL
-- 多表查询
SELECT s.`studentno`,`studentname`,`subjectname`,`studentresult`
FROM student s
RIGHT JOIN `result` r
ON s.`studentno`=r.`studentno`
INNER JOIN `subject` sub
ON r.`studentno`=sub.`subjectno`