A brief discussion on ifnull() function similar to nvl() function in MySQL

IFNULL(expr1,expr2)

If expr1 is not NULL, IFNULL() returns expr1, otherwise it returns expr2. IFNULL() returns a numeric or string value, depending on the context in which it is used.

mysql> select IFNULL(1,0);
    -> 1
mysql> select IFNULL(0,10);
    -> 0
mysql> select IFNULL(1/0,10);
    -> 10
mysql> select IFNULL(1/0,'yes');
    -> 'yes'
 
IF(expr1,expr2,expr3)

If expr1 is TRUE (expr1<>0 and expr1<>NULL), then IF() returns expr2, otherwise it returns expr3. IF() returns a number or string value, depending on the context in which it is used.

mysql> select IF(1>2,2,3);
    -> 3
mysql> select IF(1<2,'yes','no');
    -> 'yes'
mysql> select IF(strcmp('test','test1'),'yes','no');
    -> 'no'

expr1 is evaluated as an integer value, which means that if you are testing a floating point or string value, you should use a comparison operator to do so.

mysql> select IF(0.1,1,0);
    -> 0
mysql> select IF(0.1<>0,1,0);
    -> 1

In the first case above, IF(0.1) returns 0 because 0.1 is converted to an integer value, resulting in the test IF(0). This might not be what you expected. In the second case, the comparison tests the original floating-point value to see if it is nonzero, and the result of the comparison is used as an integer.

CASE value WHEN [compare-value] THEN result [WHEN [compare-value] THEN result ...] [ELSE result] END 
　 
CASE WHEN [condition] THEN result [WHEN [condition] THEN result ...] [ELSE result] END 

The first version returns result where value = compare-value. The second version returns result if the first condition is true. If there is no matching result value, the result after ELSE is returned. If there is no ELSE part, then NULL is returned.

mysql> SELECT CASE 1 WHEN 1 THEN "one" WHEN 2 THEN "two" ELSE "more" END;
    -> "one"
mysql> SELECT CASE WHEN 1>0 THEN "true" ELSE "false" END;
    -> "true"
mysql> SELECT CASE BINARY "B" when "a" then 1 when "b" then 2 END;
-> NULL

The above is a brief discussion on the ifnull() function in MySQL which is similar to the nvl() function. This is all I want to share with you. I hope it can give you a reference. I also hope that you will support 123WORDPRESS.COM.