Ask a mysql question. obtain the maximum value of a field in the query result (sort by other fields)

Ask a mysql question. I am very embarrassed to obtain the maximum value of a field in the query result (sorted by other fields... however, it is often too late to go to the manual to find a solution... I had to extend my hand...

This is the problem. perform a regular query and sort by field a in reverse order. However, you want to obtain the maximum value of field B in this query.

Php does not seem to provide a function to directly obtain the maximum key value of a multi-dimensional array .. it's not impossible to write and sort by yourself, but I still want to ask if I can find it directly on the mysql side (and there cannot be too much performance loss; otherwise, I 'd better use php to calculate it)

Thank you.

Reply to discussion (solution)

Mysql provides max functions, but this is a clustering function and is not suitable for your scenarios.

Isn't there a loop to retrieve data? In the Loop, is it okay if I use the max function?

Mysql provides max functions, but this is a clustering function and is not suitable for your scenarios.

Isn't there a loop to retrieve data? In the Loop, is it okay if I use the max function?

Well, if mysql is inconvenient, use php for computation. Currently, I have extracted all the values of field B to an array, then arsort (), and then the first one.

$ M = 0;
While ($ row = mysql_assoc ($ rs )){
$ Res [] = $ row; // This is the usual data reception
$ M = max ($ m, $ row ['B']); // calculate the maximum value of column B by the way.
}

?

Q: Will the sorting of multiple where conditions during mysql Query affect the query efficiency?

Assume there are 1 million records

Will the efficiency change if I swaps the positions of the fields id> 500000 and B <>?

Whether or not an index is critical