Join table twice - on two different columns of the same table

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

- id

- id
- table1_id
- table3_id_1
- table3_id_2

- 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: | | 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?


SELECT t2.table1_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 = t2.table3_id_1
LEFT   JOIN table3 t32 ON = 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. needs to be UNIQUE, or we might multiply rows with several matches from each LEFT JOIN:

If you join a table several times, use aliases to distinguish them:

FROM table1
JOIN table2 ON
JOIN table3 t3_1 ON
JOIN table3 t3_2 ON
WHERE ... AND ...

select as table1_id, as table2_id, 
    t3_1.value as X, 
    t3_2.value as Y
from Table1 t1
inner join Table2 t2 on = t2.table1_id
inner join Table3 t3_1 on t2.table3_id_1 =
inner join Table3 t3_2 on t2.table3_id_2 =
where t3_1.value = 'some_value'
    or t3_2.value = 'some_other_value'

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