IN语句改写EXISTS

-- IN
SELECT
    T1.*
FROM
    role_menu T1
WHERE
    T1.ROLEUUID IN (
        SELECT
            T2.uuid
        FROM
            role T2
        WHERE
            T2.UUID = ''
    );
 
-- EXISTS
SELECT
    T1.*
FROM
    role_menu t1
WHERE
    EXISTS (
        SELECT
            T2.*
        FROM
            role T2
        WHERE
            t1.ROLEUUID = T2.uuid
        AND t2.uuid = ''
    );

IN 是结果集中删选数据

EXISTS 是判断是否存在

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两三个条件使用or

多个使用IN

数据量大使用EXISTS

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