Question

这是一个查询示例：

SELECT ITEM.ID
     , DAT1.DATE
     , DAT2.DATE
     , DAT3.DATE
FROM ITEM 
   LEFT JOIN DATES AS DAT1 ON DAT1.ITEM_ID = ITEM.ID
   LEFT JOIN DATES AS DAT2 ON DAT2.ITEM_ID = ITEM.ID
   LEFT JOIN DATES AS DAT3 ON DAT3.ITEM_ID = ITEM.ID
WHERE ITEM.ID > 2000
   AND DAT1.FUNCTCODE ='START'
   AND DAT2.FUNCTCODE ='END'
   AND DAT3.FUNCTCODE ='REFURBISHED'

是否可以始终获得ID高于2000 AND的ITEMS 使用＆＃39;功能代码显示日期＆＃39;（1,2或3个字段）如果功能代码不存在行，则显示NULL？

由于

Answer 1

将where条件除第一个表格外的所有条件移至on条款：

SELECT ITEM.ID, DAT1.DATE, DAT2.DATE, DAT3.DATE
FROM ITEM 
LEFT JOIN DATES AS DAT1 ON DAT1.ITEM_ID = ITEM.ID AND DAT1.FUNCTCODE ='START'
LEFT JOIN DATES AS DAT2 ON DAT2.ITEM_ID = ITEM.ID AND DAT2.FUNCTCODE ='END'
LEFT JOIN DATES AS DAT3 ON DAT3.ITEM_ID = ITEM.ID AND  DAT3.FUNCTCODE ='REFURBISHED'
WHERE ITEM.ID > 2000;

Answer 2

如果要查看id超过2000的每个项目，无论它是否具有所有相关日期记录，请将最后三个项目放在条件中。

SELECT ITEM.ID,DAT1.DATE,DAT2.DATE,DAT3.DATE
FROM ITEM 
LEFT JOIN DATES AS DAT1 ON DAT1.ITEM_ID = ITEM.ID
LEFT JOIN DATES AS DAT2 ON DAT2.ITEM_ID = ITEM.ID
LEFT JOIN DATES AS DAT3 ON DAT3.ITEM_ID = ITEM.ID
WHERE ITEM.ID > 2000

这将为您提供如下结果：

ID    DATE    DATE    DATE
2001  null    null    null    (item with no corresponding date records)
2002  <date>  <date>  <date>  (item with all three date records)
2003  <date>  null    null    (item with only the first date record)

SQL左边的Join没有匹配的地方

2 个答案: