text: /* ============================ < Spec  > ============================ */
  
  -- query_1
  UNION /*[ALL]*/
  -- query_2
  UNION /*[ALL]*/
  -- query_3
  -- ...;
  
  /* ============================ < Example  > ============================ */
  
  CREATE TABLE t1(
      v1 INT
  );
   
  INSERT INTO t1(v1)
  VALUES(1),(2),(3);
   
  CREATE TABLE t2(
      v2 INT
  );
  INSERT INTO t2(v2)
  VALUES(2),(3),(4);
  
  -- UNION
  SELECT v1
    FROM t1
  UNION
  SELECT v2
    FROM t2;
  
  
  -- UNION ALL
  SELECT v1
    FROM t1
  UNION ALL
  SELECT v2
    FROM t2;
  
  
  SELECT FirstName, LastName, 'Employee' AS Type
  FROM employees
  UNION
  SELECT FirstName, LastName, 'Customer'
  FROM customers;
  
  /* ============================ < UNION + ORDER BY  > ============================ */
  
  SELECT FirstName, LastName, 'Employee' AS Type
  FROM employees
  UNION
  SELECT FirstName, LastName, 'Customer'
  FROM customers
  ORDER BY FirstName, LastName;