3 MYSQL Solutions
FASTEST : Using Group BY\nSELECT T.employee_id FROM\n(\n    SELECT * FROM Employees \n    UNION ALL\n    SELECT * FROM Salaries\n) T\nGROUP BY T.employee_id\nHA
#FASTEST : Using Group BY\nSELECT T.employee_id FROM\n(\n    SELECT * FROM Employees \n    UNION ALL\n    SELECT * FROM Salaries\n) T\nGROUP BY T.employee_id\nHAVING COUNT(T.employee_id) = 1 \nORDER BY T.employee_id ASC;\n\n#Using UNION and WHERE NOT IN\nSELECT employee_id FROM Salaries WHERE employee_id NOT IN (SELECT employee_id FROM Employees WHERE name IS NOT NULL)\nUNION\nSELECT employee_id FROM Employees WHERE employee_id NOT IN (SELECT employee_id FROM Salaries WHERE salary IS NOT NULL)\nORDER BY employee_id ASC;\n\n#Using JOIN\nSELECT JOINEDTABLE.employee_id\nFROM\n    (   \n        SELECT *\n        FROM Employees\n        LEFT JOIN Salaries\n        USING(employee_id)\n    UNION\n        SELECT * \n        FROM Salaries\n        LEFT JOIN Employees\n        USING(employee_id)\n    )\n    AS JOINEDTABLE\nWHERE JOINEDTABLE.salary IS NULL OR JOINEDTABLE.name IS NULL\nORDER BY JOINEDTABLE.employee_id ASC;
['Union Find', 'MySQL']