Error 111, invalid use of group functions

Question

SELECT DISTINCT CONCAT('TOTAL NUMBER OF LEGS IN A TRIP NUMBER ', T1.TNUM, ' IS NOT EQUAL TO THE LARGEST NUMBER OF LEG IN THE TRIP') AS 'ERR'
FROM TRIPLEG T1
WHERE MAX(T1.LEGNUM) = (SELECT COUNT(T2.TNUM) 
                    FROM TRIPLEG
                    WHERE T1.TNUM = T2.TNUM);

with my code, i am trying to finds all trips that violate the following consistency constraint. the last leg of each trip must be equal to the total number of legs in a trip"

CREATE TABLE TRIPLEG(
TNUM    DECIMAL(10)     NOT NULL,
LEGNUM  DECIMAL(2)  NOT NULL,
DEPARTURE VARCHAR(30)   NOT NULL,     DESTINATION VARCHAR(30) NOT NULL,
    CONSTRAINT TRIPLEG_PKEY PRIMARY KEY (TNUM, LEGNUM),
    CONSTRAINT TRIPLEG_UNIQUE UNIQUE(TNUM, DEPARTURE, DESTINATION),
CONSTRAINT TRIPLEG_FKEY1 FOREIGN KEY (TNUM) REFERENCES TRIP(TNUM) );

INSERT INTO TRIPLEG VALUES( 1, 1, 'Sydney', 'Melbourne');
INSERT INTO TRIPLEG VALUES( 1, 2, 'Melbourne', 'Hobart');

Answer 1

Try following;)

SELECT DISTINCT CONCAT('TOTAL NUMBER OF LEGS IN A TRIP NUMBER ', T1.TNUM, ' IS NOT EQUAL TO THE LARGEST NUMBER OF LEG IN THE TRIP') AS 'ERR'
FROM TRIPLEG T1
GROUP BY T1.TNUM
HAVING COUNT(T1.TNUM) <> MAX(T1.LEGNUM)