表联接

表联接 多表联接用JOIN 关键字，
标准结构：SELECT 列1，列2 form表1 JOIN 表2 ON 表1.列 = 表2.列；
ON 可以看成WHERE子句

-- 效率很低，外层查一次，里层就要查一次
SELECT s.name ,(SELECT c.name FROM classes c WHERE s.classid = c.id)AS '班级' FROM sudent s;

-- 表联接
内联，INNER JOIN，一般省略写成JOIN
SELECT s.name,c.name FROM sudent s JOIN classes c ON s.classId = c.id;
SELECT s.name,c.name FROM sudent s INNER JOIN classes c ON s.classId = c.id;

SELECT s.* FROM sudent s WHERE s.classId = (SELECT c.id FROM classes c WHERE c.name = '1班');
SELECT s.* FROM sudent s JOIN classes c ON c.name = '1班' AND s.classId=c.id;

内联接简便写法：推荐 写JOIN ON
SELECT s.name,c.name FROM sudent s,classes c WHERE s.classid = c.id AND c.name = '1班';

外联接，左外联接，右外联接,
这是一种不太严谨的联接，有的数据都会查出来，不管ON是否匹配

SELECT s.*,c.id FROM sudent s RIGHT JOIN classes c ON c.name = '1班' AND s.classId=c.id;
SELECT s.*,c.id FROM sudent s RIGHT JOIN classes c ON s.classId=c.id;
SELECT s.*,c.id FROM sudent s LEFT JOIN classes c ON s.classId=c.id;