PostgreSQL同表左外连接多表答案

【问题标题】：PostgreSQL Same Table Left Outer Join With Multiple TablesPostgreSQL同表左外连接多表
【发布时间】：2023-03-11 09:46:01
【问题描述】：

我有一张表需要与两个不同的表进行左外连接。当我将表两次放入查询中并在 where 子句中将其与自身连接时（如下面的句子），它可以工作。我认为这不应该是正确的方法。如何编写 select 语句并将表与多个不同的表进行外连接？

SELECT cols."COLUMN_NAME"
     , COALESCE(translations."COLUMN_LANG_TITLE",cols."COLUMN_TITLE") AS "COLUMN_TITLE" 
     , options."OPTION_DESC"
  FROM 

"EXTAPP_SETUP"."OBJECT_COLUMNS_TAB" cols LEFT OUTER JOIN "EXTAPP_SETUP"."COLUMN_LANG_TITLES_TAB" translations
        ON translations."EXTAPP_ID" = cols."EXTAPP_ID" and translations."OBJECT_NAME" = cols."OBJECT_NAME" and translations."COLUMN_NAME" = cols."COLUMN_NAME" and translations."LANGUAGE_CODE" = 'fr',

"EXTAPP_SETUP"."OBJECT_COLUMNS_TAB" cols2 LEFT OUTER JOIN "EXTAPP_SETUP"."COL_FIX_OPTIONS_TAB" options
        ON options."EXTAPP_ID" = cols2."EXTAPP_ID" and options."OBJECT_NAME" = cols2."OBJECT_NAME" and options."COLUMN_NAME" = cols2."COLUMN_NAME" 

WHERE cols."EXTAPP_ID" = cols2."EXTAPP_ID" and cols."OBJECT_NAME" = cols2."OBJECT_NAME" and cols."COLUMN_NAME" = cols2."COLUMN_NAME"

【问题讨论】：

标签： postgresql left-join

【解决方案1】：

您可以使用后续的左连接。由于您并没有真正使用 cols2 表，因此您可以从查询中消除它。这也将消除 where 子句，因为您不需要将 cols 表与其自身连接

SELECT cols."COLUMN_NAME"
  , COALESCE(translations."COLUMN_LANG_TITLE",cols."COLUMN_TITLE") AS "COLUMN_TITLE" 
  , options."OPTION_DESC"
FROM "EXTAPP_SETUP"."OBJECT_COLUMNS_TAB" cols 
LEFT OUTER JOIN "EXTAPP_SETUP"."COLUMN_LANG_TITLES_TAB" translations
  ON translations."EXTAPP_ID" = cols."EXTAPP_ID" 
    and translations."OBJECT_NAME" = cols."OBJECT_NAME" 
    and translations."COLUMN_NAME" = cols."COLUMN_NAME" 
    and translations."LANGUAGE_CODE" = 'fr' 
LEFT OUTER JOIN "EXTAPP_SETUP"."COL_FIX_OPTIONS_TAB" options
  ON options."EXTAPP_ID" = cols."EXTAPP_ID" 
    and options."OBJECT_NAME" = cols."OBJECT_NAME" 
    and options."COLUMN_NAME" = cols."COLUMN_NAME"

【讨论】：