Question

I have a table that contains common entries in two columns.

For Example:

Column1    Column2
Entry1  || NULL  
Entry2  || Entry1
Entry3  || Entry1
Entry4  || Entry4
Entry5  || NULL

I want to find how many times the entries of column 1 appear in column 2.

So the result would be something like:

Column1    Count
Entry1  || 2
Entry2  || 0
Entry3  || 0
Entry4  || 1
Entry5  || 0

Answer 1

WITH counts AS (
    SELECT column2, COUNT(*) AS the_count
      FROM x
      GROUP BY column2
)
SELECT x.column1, COALESCE(c.the_count, 0)
  FROM x LEFT OUTER JOIN counts c ON (x.column1 = c.column2)
  ORDER BY 1