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  • 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;

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  • 原文地址:https://www.cnblogs.com/chenyuanqiu2008/p/5464000.html
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