mysql

SHOW COLUMNS FROM  tbl_desc WHERE lower(Field) NOT IN ('id');

GROUP_CONCAT: group后连接同一列数据

CONCAT, CONCAT_WS: 连接不同列数据

取前3，4，5条数据: select * from t_book t where  (select count(1) from t_book t1 where t.bookId > t1.bookId) BETWEEN 2 and 4

转数字: select CONVERT('1a', SIGNED), CAST('1a' as SIGNED),CONVERT('1a', DECIMAL)

if(condition, true, false), coalesce(col, null('a')|else(col))