SQL JOIN: how to do?

Question

Say I have the following tables:

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table_one
================
- table_one_id (PK)
- field_1
- field_2


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table_map
================
- table_one_id
- table_two_id


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table_two
================
- table_two_id (PK)
- field_1
- field_2

Given the value table_two_id , I need all the records in table_one joined with the columns in table_two WHERE table_one->table_one_id = table_map->table_one_id, table_map->table_two_id = table_two->table_two_id.

Answer 1

you can do it with sub query like

  select * from table_one where table_one_id in 
    (select table_one_id from table_map where table_tow_id = my_value)

or with 2 joins like other suggest

Answer 2

SELECT t1.* FROM table_one t1 
   JOIN table_map t2 ON t1.table_one_id = t2.table_one_id 
   WHERE t2.table_two_id = X 

Answer 3

SELECT t1.field_1 as t1_field1, t2.field_1 as t2_field2
FROM table_one t1, table_map m, table_two t2
WHERE
    m.table_two_id=123
AND t1.table_one_id=m.table_one_id
AND t2.table_two_id=m.table_two_id

Something like that will give you access to the first field in both tables, simply extend to which fields you want

Answer 4

SELECT * 
FROM table_one 
CROSS JOIN table_map USING(table_one_id)
CROSS JOIN table_two USING(table_two_id);

This will select all rows from table1,table2,table3 that have matching rows. If you need to select ALL rows from table_one and then try to find some matches in table_two through table_map, use this:

SELECT * 
FROM table_one 
LEFT JOIN table_map USING(table_one_id)
CROSS JOIN table_two USING(table_two_id);

— all rows from table_one. When no matching row found — you'll get NULLs in corresponding columns.

USING means the query will use columns with matching names:

USING(table_one_id)

and is equivalent to:

ON(table_one.table_one_id = table_map.table_one_id)