使用SQL联接和分组方式时如何计算行数

Question

I have the following query:

SELECT a.HotelID,a.Hotelname,GROUP_CONCAT(DISTINCT b.OperatorName) AS Operators
FROM hotels AS a
INNER JOIN operators AS b
ON a.HotelID = b.HotelID
GROUP BY a.HotelID
ORDER BY a.HotelID
LIMIT 100

I need this query for a simple search function. The result Table should contain Paging. So what I did was I runned this query (without LIMIT) to get the number of rows (which I need to calculate the pages and so on) and then I rerun that query with the LIMIT.

In fact the query itself takes 4-5sec (against 300k table, with indexes on all the fields) which means it currently takes 10sec to load because it runs two times.

I am wondering if there is a SQL Statement I can simply use to get the number of rows and which might be faster. I thought I can use COUNT(a.HotelID) but this not works.

Answer 1

update

select count(*) from (
SELECT distinct b.HotelID
FROM hotels AS a
INNER JOIN operators AS b
ON a.HotelID = b.HotelID    
)

can this be faster?

Answer 2

give this a try:

SELECT *
FROM (
    SELECT a.HotelID,a.Hotelname,GROUP_CONCAT(DISTINCT b.OperatorName) AS Operators, COUNT(a.HotelID) AS total
    FROM hotels AS a
    INNER JOIN operators AS b
    ON a.HotelID = b.HotelID
    GROUP BY a.HotelID
    ) AS a
ORDER BY a.HotelID
LIMIT 100

also, for the speed you should make sure your indexes are in order.

Answer 3

Clearly described in the manual :

SQL_CALC_FOUND_ROWS tells MySQL to calculate how many rows there would be in the result set, disregarding any LIMIT clause. The number of rows can then be retrieved with SELECT FOUND_ROWS() . See Section 11.13, “Information Functions”.

If you follow the link to Section 11.13, there's then an example:

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:

 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.

In the absence of the SQL_CALC_FOUND_ROWS option in the most recent successful SELECT statement, FOUND_ROWS() returns the number of rows in the result set returned by that statement. If the statement includes a LIMIT clause, FOUND_ROWS() returns the number of rows up to the limit. For example, FOUND_ROWS() returns 10 or 60 , respectively, if the statement includes LIMIT 10 or LIMIT 50, 10 .

Please, use the documentation as your first port of call.