When to use LEFT JOIN and when to use INNER JOIN?

An INNER JOIN will only return the rows where there are matching values in both tables, whereas a LEFT JOIN will return ALL the rows from the LEFT table even if there is no matching row in the RIGHT table

A quick example

TableA
ID   Value
1    TableA.Value1
2    TableA.Value2
3    TableA.Value3

TableB
ID   Value
2    TableB.ValueB
3    TableB.ValueC

An INNER JOIN produces:

SELECT a.ID,a.Value,b.ID,b.Value 
FROM TableA a INNER JOIN TableB b ON b.ID = a.ID

a.ID    a.Value            b.ID    b.Value
2       TableA.Value2      2       TableB.ValueB
3       TableA.Value3      3       TableB.ValueC

A LEFT JOIN produces:

SELECT a.ID,a.Value,b.ID,b.Value 
FROM TableA a LEFT JOIN TableB b ON b.ID = a.ID

a.ID    a.Value            b.ID    b.Value
1       TableA.Value1      NULL    NULL
2       TableA.Value2      2       TableB.ValueB
3       TableA.Value3      3       TableB.ValueC

As you can see, the LEFT JOIN includes the row from TableA where ID = 1 even though there's no matching row in TableB where ID = 1, whereas the INNER JOIN excludes the row specifically because there's no matching row in TableB

HTH

Is there any catch? Yes there is -- left joins are a form of outer join, while inner joins are a form of, well, inner join.

Here's examples that show the difference. We'll start with the base data:

mysql> select * from j1;
+----+------------+
| id | thing      |
+----+------------+
|  1 | hi         |
|  2 | hello      |
|  3 | guten tag  |
|  4 | ciao       |
|  5 | buongiorno |
+----+------------+

mysql> select * from j2;
+----+-----------+
| id | thing     |
+----+-----------+
|  1 | bye       |
|  3 | tschau    |
|  4 | au revoir |
|  6 | so long   |
|  7 | tschuessi |
+----+-----------+

And here we'll see the difference between an inner join and a left join:

mysql> select * from j1 inner join j2 on j1.id = j2.id;
+----+-----------+----+-----------+
| id | thing     | id | thing     |
+----+-----------+----+-----------+
|  1 | hi        |  1 | bye       |
|  3 | guten tag |  3 | tschau    |
|  4 | ciao      |  4 | au revoir |
+----+-----------+----+-----------+

Hmm, 3 rows.

mysql> select * from j1 left join j2 on j1.id = j2.id;
+----+------------+------+-----------+
| id | thing      | id   | thing     |
+----+------------+------+-----------+
|  1 | hi         |    1 | bye       |
|  2 | hello      | NULL | NULL      |
|  3 | guten tag  |    3 | tschau    |
|  4 | ciao       |    4 | au revoir |
|  5 | buongiorno | NULL | NULL      |
+----+------------+------+-----------+

Wow, 5 rows! What happened?

Outer joins such as left join preserve rows that don't match -- so rows with id 2 and 5 are preserved by the left join query. The remaining columns are filled in with NULL.

In other words, left and inner joins are not interchangeable.

Here's a rough answer, that is sort of how I think about joins. Hoping this will be more helpful than a very precise answer due to the aforementioned math issues... ;-)

Inner joins narrow down the set of rows returns. Outer joins (left or right) don't change number of rows returned, but just "pick up" additional columns if possible.

In your first example, the result will be rows from AECounts that match the conditions specified to the 1_low_level_term table. Then for those rows, it tries to join to 1_pref_term and 1_soc_term. But if there's no match, the rows remain and the joined in columns are null.

Use an inner join when you want only the results that appear in both tables that matches the Join condition.

Use a left join when you want all the results from Table A, but if Table B has data relevant to some of Table A's records, then you also want to use that data in the same query.

Use a full join when you want all the results from both Tables.