Daniel Black added a comment - 2024-10-11 06:18 ifnull appears to be taking the common type of its arguments, and then casting the expressions to that. To the first, Like MDEV-35121 - 'i' || 'o' is 0. As its in a case statement with a string its actually "0". By space(5) as a string is not-null so 5 spaces is the answer. The second case its a 0 as a number. From warnings it evaluated the spaces to not-null and the tried to convert to decimal for output. Interesting on 10.5 I got 5 warnings. 11.5 warnings from select nullif(SPACE(5), ('l' +---------+------+--------------------------------------------+ | Level | Code | Message | +---------+------+--------------------------------------------+ | Warning | 1292 | Truncated incorrect DECIMAL value: ' ' | | Warning | 1292 | Truncated incorrect DOUBLE value: 'l' | | Warning | 1292 | Truncated incorrect DOUBLE value: 'o' | +---------+------+--------------------------------------------+ This is the same result as below. "dog" is not null, but as a decimal its NULL. 10.5.27-5673cbe0941e105eb73b5ecf651a529f8e717b52 MariaDB [(none)]> select nullif("dog", 0) as c; Field 1: `c` Org_field: `` Catalog: `def` Database: `` Table: `` Org_table: `` Type: VAR_STRING Collation: utf8_general_ci (33) Length: 9 Max_length: 0 Decimals: 39 Flags:     +------+ | c | +------+ | NULL | +------+ 1 row in set, 1 warning (0.001 sec)   MariaDB [(none)]> show warnings; +---------+------+------------------------------------------+ | Level | Code | Message | +---------+------+------------------------------------------+ | Warning | 1292 | Truncated incorrect DECIMAL value: 'dog' | +---------+------+------------------------------------------+ 1 row in set (0.001 sec)