Question

I’m pretty new to SQL, and I’m trying to do something like this:

I have a tabled called Foo with two columns in it: FooId (the primary key) and BarId (a non-nullable foreign key that can have duplicate values in it).

I want to write a query that returns the BarIds that only appear once in Foo. I also want to write another query that returns the BarIds that appear more than once in Foo.

I started with something like this:

SELECT
    BarId,
    COUNT(BarId) AS "BarCount"
    FROM [Database].[dbo].[Foo]
    GROUP BY BarId

Which returns the number of occurrences of each BarId. But as soon as I try to add a WHERE clause, I get a syntax error:

SELECT
    BarId,
    COUNT(BarId) AS "BarCount"
    FROM [Database].[dbo].[Foo]
    GROUP BY BarId
    WHERE "BarCount" > 1

I’m sure this is some silly mistake that I am making by not understanding SQL very well. What am I doing wrong?

Answer 1

When used with aggregates, you need to use a HAVING clause instead of a WHERE clause:

SELECT
    BarId,
    COUNT(BarId) AS "BarCount"
    FROM [Database].[dbo].[Foo]
    GROUP BY BarId
    HAVING COUNT(BarID) > 1