關係運算符

關係運算符比較兩個表達式或值，並返回一個布爾結果。下表列出了所有PL/SQL支持的關係運算符。假設變量A=10，變量B=20，則：

運算符	描述	示例
=	檢查兩個操作數的值是否相等，如果是的話那麼條件為真。	(A = B) 結果為 false.
!= <> ~=	檢查兩個操作數的值是否相等，如果值不相等，則條件變為真。	(A != B) 結果為 true.
>	檢查左邊的操作數的值是否大於右操作數的值，如果是的話那麼條件為真。	(A > B) 結果為 false.
<	檢查左邊的操作數的值是否小於右操作數的值，如果是的話那麼條件為真。	(A < B) 結果為 true.
>=	檢查左邊的操作數的值是否大於或等於右操作數的值，如果是的話那麼條件為真。	(A >= B)  結果為 false.
<=	檢查左邊的操作數的值是否小於或等於右操作數的值，如果是的話那麼條件為真。	(A <= B) 結果為 true.

示例:

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;
/

當上述代碼在SQL提示符執行時，它產生了以下結果：

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