This script returns 2:

SET sql_mode=ORACLE;

SELECT DECODE(NULL,NULL,1,2) AS a FROM DUAL;

+------+

| a    |

+------+

|    2 |

+------+

DECODE is internally implemented using Item_func_case, which treats two NULL values as not equal, so therefore the default value 2 is returned.

Oracle database works differently:

In a DECODE function, Oracle considers two nulls to be equivalent. If expr is null, then Oracle returns the result of the first search that is also null.

The above query returns 1 in Oracle:

SELECT DECODE(NULL,NULL,1,2) AS a FROM DUAL;

	 A

----------

	 1

This script demonstrates the same problem, now using a table column instead of an explicit NULL literal:

SET sql_mode=ORACLE;

DROP TABLE t1;

CREATE TABLE t1 (a VARCHAR(10));

INSERT INTO t1 VALUES (NULL);

SELECT a, DECODE(a,NULL,1,2) AS da FROM t1;

MariaDB returns:

+------+------+

| a    | da   |

+------+------+

| NULL |    2 |

+------+------+

Oracle database returns:

A		   DA

---------- ----------

		    1

DECODE should be fixed to treat two NULL values as equivalent.