Question

由于我是初学者，我需要知道一件事。我在这样的php中有一个查询..这个查询的最佳性能实现是什么：

    select Sub_Code,Sub_Name from subj_mast where Dept_id=
       (select Dep_Id from academic where Reg_No=$reg)
       and Sem='$sem' and (Sub_Type='core' or subj_mast.Sub_Code=
(select Elective_code from elective where Reg_No=$reg and Sem='$sem'))
        or subj_mast.Sub_Code
      (select Sub_Code from history_of_arrear where Reg_No=$reg)

实际上这个查询运行良好，但我认为这会使检索变慢.. 帮助我...

Answer 1

好的，经过一些重新安排后，我尽可能地清理并构建了SQL，我得到了这个：

SELECT
    Sub_Code,
    Sub_Name
FROM subj_mast
WHERE
    Dept_id = (
        SELECT
            Dep_Id
        FROM academic
        WHERE
            Reg_No=$reg
    )
    AND
    Sem='$sem'
    AND (
        Sub_Type='core'
        OR
        subj_mast.Sub_Code=(
            SELECT
                Elective_code
            FROM elective
            WHERE
                Reg_No=$reg
                AND
                Sem='$sem'
        )
    )
    OR
    subj_mast.Sub_Code = (
        SELECT
            Sub_Code
        FROM history_of_arrear
        WHERE
            Reg_No=$reg
    )

将子查询移动到连接中，并尽可能地优化它们，而无需访问数据库和数据的测试副本，这就是我想出的：

SELECT
    S.Sub_Code,
    S.Sub_Name
FROM subj_mast S
LEFT JOIN academic A
  ON A.Dep_id = S.Dept_Id

LEFT JOIN history_of_arrear H
  ON H.Sub_Code = S.Sub_Code
     AND
     H.Reg_No=A.Reg_No

LEFT JOIN Elective_code E
  ON E.Sub_Code = S.Sub_Code
     AND
     E.Reg_No=$reg
     AND
     E.Sem='$sem'

WHERE
    S.Sem='$sem'
    AND
    A.Reg_No=$reg
    AND (
        S.Sub_Type='core'
        OR
        E.Sub_Code IS NOT NULL
    )
    OR
    H.Sub_Code IS NOT NULL

您应该创建以下索引：

CREATE INDEX idx_subjMas_dept_sem ON subj_mast(Dept_Id, Sem);
CREATE INDEX idx_academic_dep_regNo ON academic(Dep_id, Reg_No);
CREATE INDEX idx_historyOfArrear_subCode_regNo ON history_of_arrear(Sub_Code, Reg_No);
CREATE INDEX idx_ElectiveCode_sem_subCode_regNo ON history_of_arrear(Sem, Sub_Code, Reg_No);

哪个应该有助于MySQL进行这些连接。

优化一个sql select嵌套在哪里查询

1 个答案: