我的带有子查询的查询不会 return 空集

Question

我在 MySQL 中有两个 table:

Table 1 - 工单

ID	QUANTITY
1	2
2	1

Table 2 - 工作订单中的项目

ID	WORKORDER
1	1
2	1
3	2

我有一个问题：

SELECT WORKORDERS.ID
      , WORKORDERS.QUANTITY AS NOMINAL_QTY
      , COUNT(ITEMSINWORKORDERS.WORKORDER) AS ENTERED_QTY 
FROM  WORKORDERS JOIN ITEMSINWORKORDERS ON 
         ITEMSINWORKORDERS.WORKORDER = WORKORDERS.ID 
WHERE WORKORDERS.QUANTITY > 
        ( SELECT COUNT(ITEMSINWORKORDERS.WORKORDER ) 
          FROM   WORKORDERS INNER JOIN ITEMSINWORKORDERS ON 
                    ITEMSINWORKORDERS.WORKORDER = WORKORDERS.ID 
        )

最初，我的语句中有一个左外连接，所以我将其切换为内连接，希望得到一个空集。当没有缺少条目的工单时，任何人都有关于将其 return 设为空集的建议吗？它旨在查找所有未输入所有项目的工作订单，并且在 WORKORDERS table 中输入的标称数量大于 ITEMSINWORKORDERS 中与该工作订单对应的记录数。我希望 return 一个空集。相反，我得到

ID	NOMINAL_QTY	ENTERED_QTY
NULL	NULL	0

最初，我的语句中有一个左外连接，所以我将其切换为内连接，希望得到一个空集。

附录： 我尝试使用 NULLIF 自己解决这个问题：

SELECT WORKORDERS.ID
    , WORKORDERS.QUANTITY AS NOMINAL_QTY
    , NULLIF(COUNT(ITEMSINWORKORDERS.WORKORDER), 0) AS ENTERED_QTY 
FROM WORKORDERS JOIN ITEMSINWORKORDERS ON 
    ITEMSINWORKORDERS.WORKORDER = WORKORDERS.ID 
WHERE WORKORDERS.QUANTITY > 
    ( SELECT COUNT( ITEMSINWORKORDERS.WORKORDER ) 
    FROM WORKORDERS INNER JOIN ITEMSINWORKORDERS ON 
        ITEMSINWORKORDERS.WORKORDER = WORKORDERS.ID 
    )

我得到的结果令人沮丧：

ID	NOMINAL_QTY	ENTERED_QTY
NULL	NULL	NULL

Answer 1

我不知道你期望什么，因为你的查询没有提供任何行。

但是为了计数你需要 GROUP BY

CREATE TABLE WORKORDERS (
  `ID` INTEGER,
  `QUANTITY` INTEGER
);

INSERT INTO WORKORDERS
  (`ID`, `QUANTITY`)
VALUES
  ('1', '2'),
  ('2', '1');

CREATE TABLE ITEMSINWORKORDERS (
  `ID` INTEGER,
  `WORKORDER` INTEGER
);

INSERT INTO ITEMSINWORKORDERS
  (`ID`, `WORKORDER`)
VALUES
  ('1', '1'),
  ('2', '1'),
  ('3', '2');

SELECT WORKORDERS.ID
      , WORKORDERS.QUANTITY AS NOMINAL_QTY
      , COUNT(ITEMSINWORKORDERS.WORKORDER) AS ENTERED_QTY 
FROM  WORKORDERS JOIN ITEMSINWORKORDERS ON 
         ITEMSINWORKORDERS.WORKORDER = WORKORDERS.ID 
WHERE WORKORDERS.QUANTITY > 
        ( SELECT COUNT(ITEMSINWORKORDERS.WORKORDER ) 
          FROM   WORKORDERS INNER JOIN ITEMSINWORKORDERS ON 
                    ITEMSINWORKORDERS.WORKORDER = WORKORDERS.ID 
        ) 
GROUP BY WORKORDERS.ID
      , WORKORDERS.QUANTITY

ID | NOMINAL_QTY | ENTERED_QTY
-: | ----------: | ----------:

SELECT COUNT(ITEMSINWORKORDERS.WORKORDER ) 
          FROM   WORKORDERS INNER JOIN ITEMSINWORKORDERS ON 
                    ITEMSINWORKORDERS.WORKORDER = WORKORDERS.ID

| COUNT(ITEMSINWORKORDERS.WORKORDER ) |
| ----------------------------------: |
|                                   3 |

db<>fiddle here

我的带有子查询的查询不会 return 空集

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