Search code examples
sqlpostgresqlsql-updatesubquerypostgresql-9.3

Using subqueries in PostgreSQL

I'm only about a day into using PostgreSQL and have what is pry a quite simple question. I want to create a left outer join to search for mismatch values. I'd like from the join create a subquery which selects the null values and then updates are based on this selection. What I have for the left outer join looks like this:

SELECT TableA.field1, TableA.field2, TableB.field3
FROM 
   TableA
LEFT OUTER JOIN TableB ON TableA.field1 = TableB.field1

This works for the join. However, I can't manage to figure out how to then use a subquery to return only rows where TableB.field1 is null.

Solution

  • You don't need a subquery:

    SELECT TableA.field1, TableA.field2, TableB.field1
FROM TableA
   LEFT OUTER JOIN TableB ON TableA.field1 = TableB.field1
where tableB.field1 IS NULL;

    Not sure what you mean with "and then updates a based on this selection" though