数据的管理在很大一部分是在进行查找工作，而SELECT占据了很大的一部分
SELECT select_expr [,select_expr...]
[
FROM table_reference
WHERE [where_condition]
[GROUP BY {col_name | position} [ASC| DESC],...]
[HAVING where_condition]
[ORDER BY {col_name | expr |position} [ASC| DESC],...]
[LIMIT {[offset,] row_count | row_cou
