一、SQL語句的執行順序舉例(sqlServer版):
(8) SELECT (9) DISTINCT (11) <TOP_specification> <select_list>
(12)INTO<new_table>
(1) FROM <left_table> (3) <join_type> JOIN <right_table> (2) ON <join_condition>
(4) WHERE <where_condition>
(5) GROUP BY <group_by_list>
(6) WITH {CUBE | ROLLUP}
(7) HAVING <having_condition>
(10) ORDER BY <order_by_list> [ASC|DESC]
二、通用SQL語句的執行順序:
(8) SELECT (9)DISTINCT<select_list>
(1) FROM <left_table>
(3) <join_type> JOIN <right_table>
(2) ON <join_condition>
(4) WHERE <where_condition>
(5) GROUP BY <group_by_list>
(6) WITH {CUBE|ROLLUP}
(7) HAVING <having_condition>
(10) ORDER BY <order_by_list>
(11) LIMIT <limit_number>
三、重新排序後的通用SQL語句的執行順序:
(1) FROM <left_table>
(2) ON <join_condition>
(3) <join_type> JOIN <right_table>
(4) WHERE <where_condition>
(5) GROUP BY <group_by_list>
(6) WITH {CUBE|ROLLUP}
(7) HAVING <having_condition>
(8) SELECT
(9) DISTINCT<select_list>
(10) ORDER BY <order_by_list>
(11) LIMIT <limit_number>
四、通用SQL語句的執行順序解釋:
- FROM:對FROM子句中的左表<left_table>和右表<right_table>執行笛卡兒積,產生虛擬表VT1;
- ON:對虛擬表VT1進行ON篩選,只有那些符合<join_condition>的行才被插入虛擬表VT2;
- JOIN:如果指定了OUTER JOIN(如LEFT OUTER JOIN、RIGHT OUTER JOIN),那麼保留表中未匹配的行作爲外部行添加到虛擬表VT2,產生虛擬表VT3。如果FROM子句包含兩個以上的表,則對上一個連接生成的結果表VT3和下一個表重複執行步驟1~步驟3,直到處理完所有的表;
- WHERE:對虛擬表VT3應用WHERE過濾條件,只有符合<where_condition>的記錄纔會被插入虛擬表VT4;
- GROUP BY:根據GROUP BY子句中的列,對VT4中的記錄進行分組操作,產生VT5;
- CUBE | ROLLUP:對VT5進行CUBE或ROLLUP操作,產生表VT6;
- HAVING:對虛擬表VT6應用HAVING過濾器,只有符合<having_condition>的記錄纔會被插入到VT7;
- SELECT:第二次執行SELECT操作,選擇指定的列,插入到虛擬表VT8中;
- DISTINCT:去除重複,產生虛擬表VT9;
- ORDER BY:將虛擬表VT9中的記錄按照<order_by_list>進行排序操作,產生虛擬表VT10;
- LIMIT:取出指定行數範圍的記錄,產生虛擬表VT11,並返回給查詢用戶;
五、以上文章內容參考以下兩篇博客,感謝原作者:
- https://blog.csdn.net/yy339452689/article/details/103958173 (Sql語句執行順序,你理解了嗎)
- https://blog.csdn.net/mango_love/article/details/44836829 (SQL server中的SELECT查詢語句執行順序)