Oracle数据库的比较运算符Comparison Operators

Comparison operators compare one expression to another. The result is always either TRUE, FALSE, or NULL.

If the value of one expression is NULL, then the result of the comparison is also NULL.

如果一个表达式的值为NULL，那么比较的结果也是NULL。

The comparison operators are:

• IS NOT NULL Operator
• Relational Operators
• LIKE Operator
• BETWEEN Operator
• IN Operator

CREATE OR REPLACE PROCEDURE print_boolean (
  b_name   VARCHAR2,
  b_value  BOOLEAN
) AUTHID DEFINER IS
BEGIN
  IF b_value IS NULL THEN
    DBMS_OUTPUT.PUT_LINE (b_name || ' = NULL');
  ELSIF b_value = TRUE THEN
    DBMS_OUTPUT.PUT_LINE (b_name || ' = TRUE');
  ELSE
    DBMS_OUTPUT.PUT_LINE (b_name || ' = FALSE');
  END IF;
END;
/
 

1. IS NOT NULL Operator

The IS NULL operator returns the BOOLEAN value TRUE if its operand is NULL or FALSE if it is not NULL. The IS NOT NULL operator does the opposite.

Comparisons involving NULL values always yield NULL.

To test whether a value is NULL, use IF value IS NULL, as in these examples:

1.1 Variable Initialized to NULL by Default

In this example, the variable counter has the initial value NULL, by default. The example uses the "IS NOT NULL Operator" to show that NULL is different from zero.

DECLARE
  counter INTEGER;  -- initial value is NULL by default
BEGIN
  counter := counter + 1;  -- NULL + 1 is still NULL
  
  IF counter IS NULL THEN
    DBMS_OUTPUT.PUT_LINE('counter is NULL.');
  END IF;
END;
/
-- run result 
counter is NULL. 

1.2 Procedure Prints BOOLEAN Variable"

DECLARE
  PROCEDURE print_x_and_y (
    x  BOOLEAN,
    y  BOOLEAN
  ) IS
  BEGIN
   print_boolean ('x', x);
   print_boolean ('y', y);
   print_boolean ('x AND y', x AND y);
 END print_x_and_y;
 
BEGIN
 print_x_and_y (FALSE, FALSE);
 print_x_and_y (TRUE, FALSE);
 print_x_and_y (FALSE, TRUE);
 print_x_and_y (TRUE, TRUE);
 
 print_x_and_y (TRUE, NULL);
 print_x_and_y (FALSE, NULL);
 print_x_and_y (NULL, TRUE);
 print_x_and_y (NULL, FALSE);
END;
/
-- run result 
x = FALSE
y = FALSE
x AND y = FALSE
x = TRUE
y = FALSE
x AND y = FALSE
x = FALSE
y = TRUE
x AND y = FALSE
x = TRUE
y = TRUE
x AND y = TRUE
x = TRUE
y = NULL
x AND y = NULL
x = FALSE
y = NULL
x AND y = FALSE
x = NULL
y = TRUE
x AND y = NULL
x = NULL
y = FALSE
x AND y = FALSE
 

1.3 Searched CASE Expression with WHEN ... IS NULL

DECLARE
  grade CHAR(1); -- NULL by default
  appraisal VARCHAR2(20);
BEGIN
  appraisal :=
    CASE
      WHEN grade IS NULL THEN 'No grade assigned'
      WHEN grade = 'A' THEN 'Excellent'
      WHEN grade = 'B' THEN 'Very Good'
      WHEN grade = 'C' THEN 'Good'
      WHEN grade = 'D' THEN 'Fair'
      WHEN grade = 'F' THEN 'Poor'
      ELSE 'No such grade'
    END;
    DBMS_OUTPUT.PUT_LINE ('Grade ' || grade || ' is ' || appraisal);
END;
/
-- run result
Grade  is No grade assigned 

2. Relational Operators

Operator Meaning

Operator	Operator
=	equal to
<>, !=, ~=, ^=	not equal to
<	less than
>	greater than
<=	less than or equal to
>=	greater than or equal to

Topics

• Arithmetic Comparisons
• BOOLEAN Comparisons
• Character Comparisons
• Date Comparisons

2.1 Relational Operators in Expressions

This example invokes the print_boolean procedure print the values of expressions that use relational operators to compare arithmetic values.

BEGIN
  print_boolean ('(2 + 2 =  4)', 2 + 2 = 4);
  
  print_boolean ('(2 + 2 <> 4)', 2 + 2 <> 4);
  print_boolean ('(2 + 2 != 4)', 2 + 2 != 4);
  print_boolean ('(2 + 2 ~= 4)', 2 + 2 ~= 4);
  print_boolean ('(2 + 2 ^= 4)', 2 + 2  ^= 4);
  
  print_boolean ('(1 < 2)', 1 < 2);
 
  print_boolean ('(1 > 2)', 1 > 2);
 
  print_boolean ('(1 <= 2)', 1 <= 2);
 
  print_boolean ('(1 >= 1)', 1 >= 1);
END;
/
-- run result
BEGIN
  print_boolean ('(2 + 2 =  4)', 2 + 2 = 4);
  
  print_boolean ('(2 + 2 <> 4)', 2 + 2 <> 4);
  print_boolean ('(2 + 2 != 4)', 2 + 2 != 4);
  print_boolean ('(2 + 2 ~= 4)', 2 + 2 ~= 4);
  print_boolean ('(2 + 2 ^= 4)', 2 + 2  ^= 4);
  
  print_boolean ('(1 < 2)', 1 < 2);
 
  print_boolean ('(1 > 2)', 1 > 2);
 
  print_boolean ('(1 <= 2)', 1 <= 2);
 
  print_boolean ('(1 >= 1)', 1 >= 1);
END;
/
  

2.2 BOOLEAN Comparisons

By definition, TRUE is greater than FALSE. Any comparison with NULL returns NULL.

2.3 Character Comparisons

By default, one character is greater than another if its binary value is larger.

For example, this expression is true:

'y' > 'r'

Strings are compared character by character. For example, this expression is true:

'Kathy' > 'Kathryn'

2.4 Date Comparisons

One date is greater than another if it is more recent.

For example, this expression is true:

'01-JAN-91' > '31-DEC-90'

3. LIKE Operator

LIKE Operator

The LIKE operator compares a character, string, or CLOB value to a pattern and returns TRUE if the value matches the pattern and FALSE if it does not.

Case is significant.

The pattern can include the two wildcard characters underscore (_) and percent sign (%).

Underscore matches exactly one character.

Percent sign (%) matches zero or more characters.

To search for the percent sign or underscore, define an escape character and put it before the percent sign or underscore.

3.1 LIKE Operator in Expression

The string 'Johnson' matches the pattern 'J%s_n' but not 'J%S_N', as this example shows.

DECLARE
  PROCEDURE compare (
    value   VARCHAR2,
    pattern VARCHAR2
  ) IS
  BEGIN
    IF value LIKE pattern THEN
      DBMS_OUTPUT.PUT_LINE ('TRUE');
    ELSE
      DBMS_OUTPUT.PUT_LINE ('FALSE');
    END IF;
  END;
BEGIN
  compare('Johnson', 'J%s_n');
  compare('Johnson', 'J%S_N');
END;
/
-- run result
TRUE
FALSE 

4. BETWEEN Operator

The BETWEEN operator tests whether a value lies in a specified range.

The value of the expression x BETWEEN a AND b is defined to be the same as the value of the expression (x>=a) AND (x<=b) . The expression x will only be evaluated once.

4.1 BETWEEN Operator in Expressions

This example invokes the print_boolean procedure print the values of expressions that include the BETWEEN operator.

BEGIN
  print_boolean ('2 BETWEEN 1 AND 3', 2 BETWEEN 1 AND 3);
  print_boolean ('2 BETWEEN 2 AND 3', 2 BETWEEN 2 AND 3);
  print_boolean ('2 BETWEEN 1 AND 2', 2 BETWEEN 1 AND 2);
  print_boolean ('2 BETWEEN 3 AND 4', 2 BETWEEN 3 AND 4);
END;
/
-- run result
2 BETWEEN 1 AND 3 = TRUE
2 BETWEEN 2 AND 3 = TRUE
2 BETWEEN 1 AND 2 = TRUE
2 BETWEEN 3 AND 4 = FALSE

PL/SQL procedure successfully completed.

5. IN Operator

The IN operator tests set membership.

x IN (set) returns TRUE only if x equals a member of set.

5.1 IN Operator in Expressions

This example invokes the print_boolean procedure print the values of expressions that include the IN operator.

DECLARE
  letter VARCHAR2(1) := 'm';
BEGIN
  print_boolean (
    'letter IN (''a'', ''b'', ''c'')',
    letter IN ('a', 'b', 'c')
  );
  print_boolean (
    'letter IN (''z'', ''m'', ''y'', ''p'')',
    letter IN ('z', 'm', 'y', 'p')
  );
END;
/
-- run result
letter IN ('a', 'b', 'c') = FALSE
letter IN ('z', 'm', 'y', 'p') = TRUE

PL/SQL procedure successfully completed.

5.2 IN Operator with Sets with NULL Values

This example shows what happens when set includes a NULL value. This invokes the print_boolean procedure

DECLARE
  a INTEGER; -- Initialized to NULL by default
  b INTEGER := 10;
  c INTEGER := 100;
BEGIN
  print_boolean ('100 IN (a, b, c)', 100 IN (a, b, c));
  print_boolean ('100 NOT IN (a, b, c)', 100 NOT IN (a, b, c));
  
  print_boolean ('100 IN (a, b)', 100 IN (a, b));
  print_boolean ('100 NOT IN (a, b)', 100 NOT IN (a, b));
 
  print_boolean ('a IN (a, b)', a IN (a, b));
  print_boolean ('a NOT IN (a, b)', a NOT IN (a, b));
END;
/
-- run result
100 IN (a, b, c) = TRUE
100 NOT IN (a, b, c) = FALSE
100 IN (a, b) = NULL
100 NOT IN (a, b) = NULL
a IN (a, b) = NULL
a NOT IN (a, b) = NULL

PL/SQL procedure successfully completed.