Question

I have a query in Mysql and I’m here to seek your inputs.

I have 2 tables T1 and T2 in mysql.

Both contain a common id column with values as shown below –

T1 - 1, 2, 3, 3, 4, 4, 5

and

T2 - 1, 2, 3, 4, 4, 5

I am trying to query table T1 to give me output in the following manner –

1, 2, 3, 4, 4, 5

If one-one relationship exists between T1 and T2 as in value 1, 2, 5 then it is selected from T1. If many-one relationship exists between T1 and T2 as in value 3 in T1 which has one value in T2 then only one value is selected from T1. many-many as in value 4 in T1 and T2 is selected as it is from T1.

Answer 1

SELECT T3.ID FROM (SELECT DISTINCT ID FROM T1) T3
INNER JOIN T2 ON T3.ID = T2.ID

It collapses the results from T1 into a distinct set, then re-inflates it based on matches found in T2.