加入表两次 - 在同一个表的两个不同列上 [英] Join table twice - on two different columns of the same table

问题描述

我有一个非常混乱的数据库，其中一个表在单独的表中保存了我需要的两个值.这是我的问题:

表1- ID表2- ID- table1_id- table3_id_1- table3_id_2表3- ID- 价值

我需要从 table1 开始并执行一个连接，以便在两个单独的列中返回来自 table3 的值.所以我想要这样的东西:

table1.id |table2.id |table2.table3_id_1 |table2.table3_id_2 |X |是

其中X和Y分别是table3_id_1和table3_id_2连接的行的值.p>

可能使它们成为变量或其他东西，以便我也可以在 WHERE 子句中过滤它们?

解决方案

SELECT t2.table1_id, t2.id AS table2_id, t2.table3_id_1, t2.table3_id_2, t31.value AS x, t32.value AS y从表 2 t2LEFT JOIN table3 t31 ON t31.id = t2.table3_id_1LEFT JOIN table3 t32 ON t32.id = t2.table3_id_2;

没有必要涉及table1.table2 有你需要的一切——假设有一个 外键约束 保证引用完整性(所有t2.table1_id 实际上都存在于table1 中).否则，您可能想要加入 table1，从而仅选择也存在于 table1 中的行.

我使用 LEFT [OUTER]JOIN (而不是 [INNER] JOIN) 加入 table3 的两个实例，原因类似:尚不清楚是否保证引用完整性 - 以及是否有任何键列可以是 NULL.[INNER] JOIN 将从结果中删除未找到匹配项的行.我假设您宁愿为任何缺少的 x 或 y 显示带有 NULL 值的此类行.

table3.id 必须是 UNIQUE，否则我们可能会将行与每个 LEFT JOIN 的多个匹配项相乘:

• 两个 SQL 左连接产生不正确的结果

I have a very confusing database with a table that holds two values I need in a separate table. Here is my issue:

Table1
- id

Table2
- id
- table1_id
- table3_id_1
- table3_id_2

Table3
- id
- value

I need to go from table1 and do a join that would give me back the value from table3 in two separate columns. So I want something like this:

table1.id | table2.id | table2.table3_id_1 | table2.table3_id_2 | X | Y

Where X and Y are the values for the row connected by table3_id_1 and table3_id_2 respectively.

Possibly make them variables or something so I can filter them in a WHERE clause as well?

解决方案

SELECT t2.table1_id
     , t2.id          AS table2_id
     , t2.table3_id_1
     , t2.table3_id_2
     , t31.value      AS x
     , t32.value      AS y
FROM   table2 t2
LEFT   JOIN table3 t31 ON t31.id = t2.table3_id_1
LEFT   JOIN table3 t32 ON t32.id = t2.table3_id_2;

There is no need to involve table1. table2 has all you need - assuming there is a foreign key constraint guaranteeing referential integrity (all t2.table1_id are actually present in table1). Else you may want to join to table1, thereby selecting only rows also present in table1.

I use LEFT [OUTER] JOIN (and not [INNER] JOIN) to join to both instances of table3 for a similar reason: it is unclear whether referential integrity is guaranteed - and whether any of the key columns can be NULL. An [INNER] JOIN would drop rows from the result where no match is found. I assume you would rather display such rows with a NULL value for any missing x or y.

table3.id needs to be UNIQUE, or we might multiply rows with several matches from each LEFT JOIN:

• Two SQL LEFT JOINS produce incorrect result

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