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Asked: 2026-05-20T04:46:53+00:00

I’m new to SQL, and find I don’t quite understand which joins to use

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I’m new to SQL, and find I don’t quite understand which joins to use when. Sub-queries also seem confusing. I have the following tables set up, and I’m trying to get a specific outcome:

`+--------------------------+   +---------------+  +---------------+  +---------------+`
`|          Table_A         |   |   Table_B     |  |   Table_C     |  |   Table_D     |`
`+--------------------------+   +---------------+  +---------------+  +---------------+`
`| id | f2 | f3 | f4 | d_id |   | a_id  | c_id  |  |  id  |   fc   |  |  id  |   fs   |`
`+--------------------------+   +---------------+  +---------------+  +---------------+`

And this is what I’m trying to get:

`+----------------------------------+`
`| a.f2 | a.f3 | a.f4 | d.fs | c.fc |`
`+----------------------------------+`

I’ve found that I can get the first four columns with the following query:

Select t1.f2, t1.f3, t1.f4, t2.fs
    From Table_A AS t1 INNER JOIN Table_D AS t2
    ON t1.d_id = t2.id;

How can I get from A.id to C.fc? I can’t figure out how to get the fifth column, let alone in conjunction with the previous query. This is about as far as I got with the final column: 

Select t1.flow_control
    FROM Table_D AS t1 INNER JOIN policy t2
    ON t1.id = t2.c_id;
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1 Answer

  1. Editorial Team

    This should do the trick:

    Select t1.f2, t1.f3, t1.f4, t2.fs, t3.fc
    From Table_A AS t1
    INNER JOIN Table_D AS t2 ON t1.d_id = t2.id
    INNER JOIN Table_B AS jointable ON t1.id = jointable.a_id
    INNER JOIN Table_C AS t3 ON jointable.c_id = t3.id;

    The first join is the one you originally had. The second one is for the ‘linking table’, which represents a HABTM relationship between Table_A and Table_C. Once you’ve joined with that, you do a third join ‘connecting’ it to Table C so you can get that ‘fc’ field. So essentially the first join is just for Table_D, and the two other JOINs get you to that last field.

    You can have multiple JOINs in a single statement that link to completely different tables if that’s what you need… it’s just like connecting the dots / following the relationships 🙂

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