Query to generate query to check foreign key constraints

gistfile1.sql

-- Generates a SQL query that validates that none of your foreign key
-- constrainted columns are invalid (values that don't exist in the column that
-- defines valid values).
-- The query returns a list of foreign key constraints and the number of
-- violations present. If you want this query to return all constraints then
-- you can lop off the 'foo WHERE count > 0' below, this'll cause it to return
-- all foreign key constraints and the number of violations, even when 0. 
SELECT 'SELECT * FROM (' || string_agg(query, ' UNION ') || ') foo WHERE count
> 0;' FROM (
  SELECT 'SELECT count(*), ' || ''''|| constraint_name || '''' || ' FROM ' || table_name || ' WHERE ' || column_name || ' NOT IN ( SELECT ' || foreign_column_name || ' FROM ' || foreign_table_name || ')' query
      FROM (
        -- This portion of the query was adapted from:
        -- https://stackoverflow.com/questions/1152260/postgres-sql-to-list-table-foreign-keys
        SELECT
          table_constraint.constraint_name,
          table_constraint.table_name,
          key_column_usage.column_name, 
          constraint_column_usage.table_name  AS foreign_table_name,
          constraint_column_usage.column_name AS foreign_column_name 
        FROM information_schema.table_constraints AS table_constraint 
        JOIN information_schema.key_column_usage        ON        table_constraint.constraint_name = key_column_usage.constraint_name
        JOIN information_schema.constraint_column_usage ON constraint_column_usage.constraint_name = key_column_usage.constraint_name
        WHERE constraint_type = 'FOREIGN KEY'
    ) foo
) bar;