DELIMITER /

CREATE OR REPLACE FUNCTION f1() RETURNS INT

BEGIN

  SET @counter= COALESCE(@counter, 0) + 1;

  RETURN @counter;

END;

/

DELIMITER ;

SET @counter=0;

SELECT f1()=1, @counter FROM seq_1_to_5;

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

| f1()=1 | @counter |

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

|      1 |        1 |

|      0 |        2 |

|      0 |        3 |

|      0 |        4 |

|      0 |        5 |

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

SELECT ROW(f1(),1) = ROW(1,1), @counter FROM seq_1_to_5;

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

| (f1(),1) = (1,1) | @counter |

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

|                0 |        7 |

|                0 |        9 |

|                0 |       11 |

|                0 |       13 |

|                0 |       15 |

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