MySQL中SQL_CALC_FOUND_ROWS的用法

1. SQL_CALC_FOUND_ROWS簡述

在不少分頁的程序中都這樣寫:

#查出符合條件的記錄總數
SELECT COUNT(*) from [table] WHERE  ......;  
#查詢當頁要顯示的數據
SELECT * FROM [table]  WHERE ...... limit M,N;

可是從Mysql4.0.0開始，咱們能夠選擇使用另一個方式：

SELECT SQL_CALC_FOUND_ROWS * FROM [table] WHERE ......  limit M, N;
SELECT FOUND_ROWS();

#SQL_CALC_FOUND_ROWS 告訴MySQL將sql所處理的行數記錄下來
#FOUND_ROWS() 則取到了這個紀錄。

雖然也是兩個語句，可是隻執行了一次主查詢，因此效率比原來要高不少。

2.SQL_CALC_FOUND_ROWS FOUND_ROWS()文檔中英對照

FOUND_ROWS()
A SELECT statement may include a LIMIT clause to restrict the number of rows the server returns
to the client. In some cases, it is desirable to know how many rows the statement would have 
returned without the LIMIT, but without running the statement again. To obtain this row count,
include a SQL_CALC_FOUND_ROWS option in the SELECT statement, and then invoke FOUND_ROWS() afterward:

SELECT語句中常常可能用LIMIT限制返回行數。有時候可能想要知道若是沒有LIMIT會返回多少行，但又不想再執行一次相同
語句。那麼，在SELECT查詢中包含SQL_CALC_FOUND_ROWS選項，而後執行FOUND_ROWS()就能夠了：

mysql> SELECT SQL_CALC_FOUND_ROWS * FROM tbl_name WHERE id > 100 LIMIT 10;

mysql> SELECT FOUND_ROWS();

The second SELECT returns a number indicating how many rows the first SELECT would have returned
had it been written without the LIMIT clause.

第二個SELECT將返回第一條SELECT若是沒有LIMIT時返回的行數。


In the absence of the SQL_CALC_FOUND_ROWS option in the most recent SELECT statement,
FOUND_ROWS() returns the number of rows in the result set returned by that statement.

若是在前一條語句中沒有使用SQL_CALC_FOUND_ROWS選項，FOUND_ROWS()將返回前一條語句實際返回的行數。

The row count available through FOUND_ROWS() is transient and not intended to be available 
past the statement following the SELECT SQL_CALC_FOUND_ROWS statement. If you need to 
refer to the value later, save it:

FOUND_ROWS()獲得的數字是臨時的，執行下一條語句就會失效。若是想要這個數字，就要將它保存下來：

mysql> SELECT SQL_CALC_FOUND_ROWS * FROM ... ;
mysql> SET @rows = FOUND_ROWS();


If you are using SELECT SQL_CALC_FOUND_ROWS, mysql must calculate how many rows are in the full
result set. However, this is faster than running the query again without LIMIT, because the 
result set need not be sent to the client.

若是使用 SELECT SQL_CALC_FOUND_ROWS，MySQL必須計算全部結果集的行數。儘管這樣，總比再執行一次不使用LIMIT
的查詢要快多了，由於結果集不須要返回客戶端。


SQL_CALC_FOUND_ROWS and FOUND_ROWS() can be useful in situations when you want to restrict the
number of rows that a query returns, but also determine the number of rows in the full result
set without running the query again.

當你想要限制查詢的返回行數的同時又想獲得查詢的完整結果集合的行數,但又不想重複執行一次查詢,那麼
SQL_CALC_FOUND_ROWS and FOUND_ROWS() 是很是有用的！

3.UNION 語句下的用法

3.1 SQL_CALC_FOUND_ROWS只能出如今UNION的第一個SELECT中,不然編譯報錯！

mysql> (select  SQL_CALC_FOUND_ROWS  * from  actor  limit 1)  union    (select SQL_CALC_FOUND_ROWS  * from  actor limit 1);
ERROR 1234 (42000): Incorrect usage/placement of 'SQL_CALC_FOUND_ROWS'

3.2 limit中 select 中的狀況

#union all 
mysql> (select  SQL_CALC_FOUND_ROWS  * from  actor  limit 1)  union all   (select * from  actor limit 1);
+----------+------------+-----------+---------------------+
| actor_id | first_name | last_name | last_update         |
+----------+------------+-----------+---------------------+
|        1 | PENELOPE   | GUINESS   | 2006-02-15 04:34:33 |
|        1 | PENELOPE   | GUINESS   | 2006-02-15 04:34:33 |
+----------+------------+-----------+---------------------+
2 rows in set (0.00 sec)

mysql> select FOUND_ROWS();
+--------------+
| FOUND_ROWS() |
+--------------+
|            2 |
+--------------+
1 row in set (0.00 sec)

union

mysql> (select  SQL_CALC_FOUND_ROWS  * from  actor  limit 1)  union    (select * from  actor limit 1);
+----------+------------+-----------+---------------------+
| actor_id | first_name | last_name | last_update         |
+----------+------------+-----------+---------------------+
|        1 | PENELOPE   | GUINESS   | 2006-02-15 04:34:33 |
+----------+------------+-----------+---------------------+
1 row in set (0.00 sec)

mysql> select FOUND_ROWS();
+--------------+
| FOUND_ROWS() |
+--------------+
|            1 |
+--------------+
1 row in set (0.00 sec)

能夠看到,limit中 select 中時,FOUND_ROWS()返回的時顯示的行數！

3.2 limit中 UNION 外的狀況

mysql> (select  SQL_CALC_FOUND_ROWS  * from  actor  )  union    (select   * from  actor ) order by actor_id limit 2  ;
+----------+------------+-----------+---------------------+
| actor_id | first_name | last_name | last_update         |
+----------+------------+-----------+---------------------+
|        1 | PENELOPE   | GUINESS   | 2006-02-15 04:34:33 |
|        2 | NICK       | WAHLBERG  | 2006-02-15 04:34:33 |
+----------+------------+-----------+---------------------+
2 rows in set (0.00 sec)

mysql> select FOUND_ROWS();
+--------------+
| FOUND_ROWS() |
+--------------+
|          200 |
+--------------+
1 row in set (0.00 sec)

mysql> (select  SQL_CALC_FOUND_ROWS  * from  actor  )  union all   (select   * from  actor ) order by actor_id limit 2  ;
+----------+------------+-----------+---------------------+
| actor_id | first_name | last_name | last_update         |
+----------+------------+-----------+---------------------+
|        1 | PENELOPE   | GUINESS   | 2006-02-15 04:34:33 |
|        1 | PENELOPE   | GUINESS   | 2006-02-15 04:34:33 |
+----------+------------+-----------+---------------------+
2 rows in set (0.01 sec)

mysql> select FOUND_ROWS();
+--------------+
| FOUND_ROWS() |
+--------------+
|          400 |
+--------------+
1 row in set (0.00 sec)

能夠看到,limit中 UNION 外時,FOUND_ROWS()返回的全部的行數！ UNION ALL 會返回2倍的行數