Question

Let’s say that I have two tables in a database: Table1 and Table2.

Table1 have the following columns:

Var1 | Var2

and Table2 have the following columns:

Var3 | Var4

I also have a constant, x.

Now, I would like to write an SQL expression that includes every row in Table1 that satisfies the following criteria:

Var1 = x
or there is a row in Table2 where Var4=Var2 and where Var3=x

How do I write this as an SQL expression?

I guess it should start with SELECT * FROM Table1 WHERE Var1=x but from there, I’m lost.

Answer 1

The magic element that you need is a UNION, which essentially appends one result set to another.

SELECT *
FROM Table1
WHERE Var1=x

UNION

SELECT *
FROM Table1
JOIN Table2 ON Table1.Var2 = Table2.Var4
WHERE Table1.Var1 <> x -- this is to avoid dups of rows that satisfy both cases
  AND Table2.Var3 = x