MySQL have several Control Flow Functions listed as below:
1) CASE
2) IF
3) IFNULL
4) NULLIF
Control function returns value based on each row processed by the query executed. Control functions can be used on SELECT, WHERE, ORDER BY and GROUP BY statements.
1) CASE
We can say this is just like the switch case in programming languages.
Sample Queries:
SELECT CASE 0 WHEN 0 THEN 'zero' WHEN 1 THEN 'one' ELSE 'no one' END;
// Below will return true
SELECT CASE WHEN 5>2 THEN 'true' ELSE 'false' END;
2) IF
This function takes the three parameters as expression, if expression one is true the it will return second parameter otherwise it will return third parameter.
Sample Queries:
// return yes
SELECT IF(1<5,'yes','no');
3) IFNULL
This function takes two parameters as a expression. And if expression one is not num then it will return expression one otherwise it will return expression two.
Sample Queries:
// Return 5
SELECT IFNULL(5,0);
// Return 10
SELECT IFNULL(NULL,10);
4) NULLIF
This function takes two parameter as a expression. It will return NULL if expr1=expr2 will return TRUE otherwise it will return expr1.
Sample Queries:
// Return NULL
SELECT NULLIF(5,5);
// Return 10
SELECT NULLIF(10,4);
Note: MySQL evaluates expr1 twice if the arguments are not equal.
