在MySQL SELECT中生成字段

SELECT field1, field2, 'value3' as field3 FROM table_name;

SELECT field1, field2, CONCAT(field1, field2) as field3 FROM table_name;

SELECT field1, field2, CASE WHEN field1 > field2 THEN 'Greater' ELSE 'Less' END as field3 FROM table_name;

SELECT field1, field2, (SELECT COUNT(*) FROM table_name WHERE field1 > field2) as field3 FROM table_name;