如何为记录计数做 ceil 并将此数字用作 sql 中的偏移量

Question

am new t sql and am trying to solve this Question: Getting the median value from the database

I am trying initially to order the table by LAT_N and then doing offset with the value of the middle, the value of the middle will be the ceil of the number of records divided by two. so for this I tried:

select round(LAT_N,4)
from STATION
order by LAT_N desc
limit 1
offset ceil ((select count(LAT_N) from STATION)/2)

but am getting the following error:

ERROR 1064 (42000) at line 1: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near '((select count(LAT_N) from STATION)/2)' at line 6

I am wondering how to retrieve the ceil as a number to use it for offset? what did I do wrong?

Answer 1

ciel() isn't a MySQL function and it isn't possible to use a subquery as the offset number (sadly)

You need (it seems) to use dynamic sql to get this to work, eg

SET @table_name:='information_schema.columns';

set @offset := (select cast(count(*)/2 as unsigned) from information_schema.columns);

SET @sql:=CONCAT('SELECT * FROM ',@table_name, ' order by column_name limit 1 offset ',@offset);

select @sql;

PREPARE dynamic_statement FROM @sql;
EXECUTE dynamic_statement;
DEALLOCATE PREPARE dynamic_statement;

note you can place the /2 and cast the value to an unsigned integer inside that subquery to calculate the midpoint.

see a working db<>fiddle here