NOT IN改为LEFT JOIN或NOT EXISTS的实现

在SQL中用NOT IN会影响性能，导致的主要原因就是索引无效，所以最好是将NOT IN改为其他方式实现。

没修改之前的NOT IN写法：

SELECT DISTINCT
    t2.a_id
FROM
    temp_b t2
WHERE
    t2.a_id NOT IN(
        SELECT DISTINCT
            t4.a_id
        FROM
            b AS t3,
            temp_b AS t4
        WHERE
            t4.a_id = t3.a_id
    )

修改为LEFT JOIN写法：

SELECT DISTINCT
    t6.a_id
FROM
    (
        SELECT
            t2.a_id,
            t5.a_id AS tempId
        FROM
            temp_b t2
        LEFT JOIN(
            SELECT DISTINCT
                t4.a_id
            FROM
                b AS t3,
                temp_b AS t4
            WHERE
                t4.a_id = t3.a_id
        )AS t5 ON t2.a_id = t5.a_id
    )AS t6
WHERE
    t6.tempId IS NULL

修改为NOT EXISTS写法：

SELECT DISTINCT
    t2.a_id
FROM
    temp_b t2
WHERE
    NOT EXISTS(
        SELECT
            1
        FROM
            (
                SELECT DISTINCT
                    t4.a_id
                FROM
                    b AS t3,
                    temp_b AS t4
                WHERE
                    t4.a_id = t3.a_id
            )AS t5
        WHERE
            t5.a_id = t2.a_id
    )