Count sum of records where value = 1

Question

I'm trying to count all the records from multiple tables that have attr3 as a value of 1.

This is what I have so far:

SELECT 'Dept' AS table_name, COUNT(*)
FROM table1, table2
WHERE table1.attr3='1' AND table2.attr3='1' 

The problem I am having is that it's showing the wrong number. It should display as 7 , but instead it's 12 .

I have to write it in such a way because attr3 is a foreign key so some solutions may come up with attr3 is ambiguous

Anyone know where I may be going wrong? Thank you

EDIT

I then want to apply the same query two more times but with different values to get separate counts:

SELECT 'Dept' AS table_name, COUNT(*)
FROM table1, table2
WHERE table1.attr3='2' AND table2.attr3='2' 

Minimizing code would be a big plus!

Answer 1

You should use UNION , not JOIN , to combine the two tables, since you're not relating rows to each other between the tables. Then use GROUP BY to get the counts for each attr3 .

SELECT attr3, COUNT(*) 
FROM (
    SELECT attr3 FROM table1
    UNION ALL
    SELECT attr3 FROM table2
) AS u
GROUP BY attr3

It might be better to do the counts in each table separately, then sum them. If you have an index on attr3 , this will be able to take advantage of it.

SELECT attr3, SUM(count)
FROM (
    SELECT attr3, COUNT(*) AS count
    FROM table1
    GROUP BY attr3
    UNION ALL
    SELECT attr3, COUNT(*)
    FROM table2
    GROUP BY attr3
) AS u
GROUP BY attr3

Make sure you use UNION ALL , not just UNION , since by default it will remove duplicates.

Answer 2

The comma operator creates a Cartesian product. That produces way more rows than you want (under most circumstances). Instead, use union all along with your condition:

SELECT 'Dept' AS table_name, COUNT(*)
FROM ((select attr3 from table1) union all
      (select attr3 from table2)
     ) t
WHERE attr3 = '1';