 <p>Your two larger SELECTS (the first 2) are returning 12 items. The last two SELECTS (the small ones) are returning 13 items. UNION must have matching columns.</p> <p>eg statement should end...</p> <pre><code>SELECT NULL, 0, 'ABC', NULL, NULL, NULL, 'ABC', NULL, NULL, NULL, NULL, NULL FROM DUAL UNION SELECT NULL, 0, 'XYZ', NULL, NULL, NULL, 'XYZ', NULL, NULL, NULL, NULL, NULL FROM DUAL </code></pre>