如何通过减少选择和联接来改进POSTGRES（SQL）查询？

Question

我敢肯定，下面的代码是更好的语法方法，那就是，下面的代码足以实现所需的结果，但是我想使用适当的语法来做到这一点，并可能删除/减少不必要的联接和选择在同一张桌子上。

（即，多次使用rg.NAME和T.NAME字段）

INSERT INTO BSHIP (ID, IS_ID, C_ID, A_A_ID, A_A_INSTID, P_ID)
         SOME_ID,
        (SELECT t.IS_ID FROM T t LEFT OUTER JOIN BSHIP ON T.IS_ID=BSHIP.IS_ID WHERE T.NAME='XYZ'),
        (SELECT t.id FROM T t LEFT OUTER JOIN BSHIP ON T.IS_ID=BSHIP.IS_ID WHERE T.NAME='XYZ'), 
        (SELECT rg.objbshps_id FROM RT_GP rg LEFT OUTER JOIN T ON rg.IS_ID=T.IS_ID WHERE T.NAME='XYZ' AND rg.NAME='ABC'), 
        (SELECT t.IS_ID FROM T t LEFT OUTER JOIN BSHIP ON T.IS_ID=BSHIP.IS_ID WHERE T.NAME='XYZ'), 
        (SELECT rg.id FROM RT_GP rg LEFT OUTER JOIN T ON rg.IS_ID=T.IS_ID WHERE T.NAME='XYZ' AND rg.NAME='ABC'), 
  ON CONFLICT DO NOTHING;

Answer 1

您似乎想要这样的东西：

INSERT INTO BSHIP (ID, IS_ID, C_ID, A_A_ID, A_A_INSTID, P_ID)
    SELECT SOME_ID, t.IS_ID, t_id
           (SELECT rg.objbshps_id FROM RT_GP rg 
WHERE rg.IS_ID = T.IS_ID AND rg.NAME = 'ABC'), 
           t.IS_ID, 
           (SELECT rg.id FROM RT_GP rg WHERE rg.IS_ID = T.IS_ID AND rg.NAME = 'ABC')
    FROM t 
    WHERE t.NAME = 'XYZ'
    ON CONFLICT DO NOTHING;

Answer 2

这对我有效，并且更清洁。

INSERT INTO BSHIP (ID, IS_ID, C_ID, A_A_ID, A_A_INSTID, P_ID)
    SOME_ID, t.IS_ID, t_id, rg.objbshps_id, t.IS_ID, rg.ID 
    FROM t, RT_GP rg
    WHERE t.NAME = 'XYZ' 
    AND rg.NAME = 'ABC'
    AND rg.IS_ID = T.IS_ID
    ON CONFLICT DO NOTHING;