mysql ORDER BY and CASE casts INT to CHAR? - mysql

Question

For a stored procedure with configurable sort order (_Sort parameter), i use code like this:
SELECT * FROM distances
ORDER BY
CASE _Sort
WHEN 1 THEN uid
WHEN 2 THEN NULL
WHEN 3 THEN name
WHEN 4 THEN NULL
WHEN 5 THEN distance
WHEN 6 THEN NULL
ELSE distance
END ASC,
CASE _Sort
WHEN 2 THEN uid
WHEN 4 THEN name
WHEN 6 THEN distance
ELSE NULL
END DESC
in which uid is INT and distance is DOUBLE.
But if _Sort = 1, uid is ordered like it was a CHAR e.g.
200
207
25
4
Same thing for distance.
Casting to unsigned and decimal did not help.
ORDER BY uid ASC does the right thing, i.e. 4, 25, 200, 207
Any idea?

Answer 1

As documented under Control Flow Functions:
The return type of a CASE expression is the compatible aggregated type of all return values
Whilst the manual does not explicitly document how the "compatible aggregated type" is determined, one can follow the source from Item_func_case::fix_length_and_dec() through agg_result_type() to item_store_type():
static Item_result item_store_type(Item_result a, Item *item,
my_bool unsigned_flag)
{
Item_result b= item->result_type();
if (a == STRING_RESULT || b == STRING_RESULT)
return STRING_RESULT;
else if (a == REAL_RESULT || b == REAL_RESULT)
return REAL_RESULT;
else if (a == DECIMAL_RESULT || b == DECIMAL_RESULT ||
unsigned_flag != item->unsigned_flag)
return DECIMAL_RESULT;
else
return INT_RESULT;
}
One can therefore see that if a single return value is a string, then the return value of the overall CASE expression will also be a string.
In your case, one presumes that name is a string; therefore the data type returned by your CASE expression is a string. Consequently, your numeric values are compared as strings and thus sorted lexicographically (hence the output that you observe).
One way of overcoming this would be to pad all numeric values to equal widths in order that sorting lexicographically will deliver the desired results: using the ZEROFILL attribute of integer-type columns will do this automatically; however, this is still pretty inefficient and you may wish to consider redesigning your logic.
For example, you could instead build a string that contains the desired SQL; then prepare and execute a statement from that string:
SET #sql := CONCAT(
'SELECT * FROM distances ORDER BY ',
CASE _Sort
WHEN 1 THEN 'uid ASC'
WHEN 2 THEN 'uid DESC'
WHEN 3 THEN 'name ASC'
WHEN 4 THEN 'name DESC'
WHEN 5 THEN 'distance ASC'
WHEN 6 THEN 'distance DESC'
ELSE 'distance ASC'
END
);
PREPARE stmt FROM #sql;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;

Answer 2

Try this solution (edited) -
SELECT * FROM distances
ORDER BY
IF(_Sort = 1, uid, 0),
IF(_Sort = 2, NULL, 0),
IF(_Sort = 3, name, 0),
IF(_Sort = 4, NULL, 0),
...