关系运算符比较两个表达式或值，并返回一个布尔结果。下表列出了所有PL/SQL支持的关系运算符。假设变量A=10，变量B=20，则：
| 运算符 | 描述 | 示例 |
|---|---|---|
| = | 检查两个操作数的值是否相等，如果是的话那么条件为真。 | (A = B) 结果为 false. |
|
!= <> ~= |
检查两个操作数的值是否相等，如果值不相等，则条件变为真。 | (A != B) 结果为 true. |
| > | 检查左边的操作数的值是否大于右操作数的值，如果是的话那么条件为真。 | (A > B) 结果为 false. |
| < | 检查左边的操作数的值是否小于右操作数的值，如果是的话那么条件为真。 | (A < B) 结果为 true. |
| >= | 检查左边的操作数的值是否大于或等于右操作数的值，如果是的话那么条件为真。 | (A >= B) 结果为 false. |
| <= | 检查左边的操作数的值是否小于或等于右操作数的值，如果是的话那么条件为真。 | (A <= B) 结果为 true. |
| DECLARE |
| a number (2) := 21; |
| b number (2) := 10; |
| BEGIN |
| IF (a = b) then |
| dbms_output.put_line('Line 1 - a is equal to b'); |
| ELSE |
| dbms_output.put_line('Line 1 - a is not equal to b'); |
| END IF; |
| IF (a < b) then |
| dbms_output.put_line('Line 2 - a is less than b'); |
| ELSE |
| dbms_output.put_line('Line 2 - a is not less than b'); |
| END IF; |
| IF ( a > b ) THEN |
| dbms_output.put_line('Line 3 - a is greater than b'); |
| ELSE |
| dbms_output.put_line('Line 3 - a is not greater than b'); |
| END IF; |
| -- Lets change value of a and b |
| a := 5; |
| b := 20; |
| IF ( a <= b ) THEN |
| dbms_output.put_line('Line 4 - a is either equal or less than b'); |
| END IF; |
| IF ( b >= a ) THEN |
| dbms_output.put_line('Line 5 - b is either equal or greater than a'); |
| END IF; |
| IF ( a <> b ) THEN |
| dbms_output.put_line('Line 6 - a is not equal to b'); |
| ELSE |
| dbms_output.put_line('Line 6 - a is equal to b'); |
| END IF; |
| END; |
| / |
当上述代码在SQL提示符执行时，它产生了以下结果：
Line 1 - a is not equal to b Line 2 - a is not less than b Line 3 - a is greater than b Line 4 - a is either equal or less than b Line 5 - b is either equal or greater than a Line 6 - a is not equal to b PL/SQL procedure successfully completed
