Question

I need to select several columns from 2 tables:

Tab1
| Col1 | Col2 | Col3

Tab2
| Col1 | Col4 | Col5

This is my query:

SELECT A.*,B.Col4 
FROM Tab1 A,Tab2 B 
WHERE A.Col1=B.Col1 
AND A.Col2=='XXX'
AND A.Col3>5;

However this query concatenates two tables in some manner and duplicates some rows. What is wrong with it?

The expected result is this one:

Tab3
| Col1 | Col2 | Col3 | Col4

Answer 1

There is no problem with the query you posted, but if the problem is that there are

duplicates some rows

As you described then use DISTINCT:

SELECT DISTINCT A.*, B.Col4 
FROM Tab1 A, Tab2 B 
WHERE A.Col1 = B.Col1 
AND A.Col2 = 'XXX'
AND A.Col3 > 5;

Or the other JOIN syntax:

SELECT DISTINCT A.*,B.Col4 
FROM Tab1 A 
INNER JOIN Tab2 B ON A.Col1 = B.Col1 
WHERE A.Col2 ='XXX'
  AND A.Col3 > 5;