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Certain complex MySQL SELECT and subquery statements will not allow the use of the * wildcard and you will need to fill in the entire column list of a given table. Consider the following simplified example, a SELECT statement that contains 3 columns. The asterisk here refers to all columns, which is actually the 3 columns listed in the GROUP BY clause:

SELECT
 IF(
  EXISTS(
  SELECT *
  FROM (
   SELECT *
   FROM `dbName_A`.`tableName_A`
   UNION ALL
   SELECT *
   FROM `dbName_B`.`tableName_B`
   AS `compareTables`
  GROUP BY `column_1`, `column_2`, `column_3`
  HAVING COUNT(*) = 1),
 1, 0);

Imagine if it were dozens of columns instead of just 3. You can't simply put in the * wildcard like 'GROUP BY * '. The above example will not …