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
BEGINIF b_value IS NULL THENDBMS_OUTPUT.PUT_LINE (b_name || ' = NULL');ELSIF b_value = TRUE THENDBMS_OUTPUT.PUT_LINE (b_name || ' = TRUE');ELSEDBMS_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.

DECLAREcounter INTEGER;  -- initial value is NULL by default
BEGINcounter := counter + 1;  -- NULL + 1 is still NULLIF counter IS NULL THENDBMS_OUTPUT.PUT_LINE('counter is NULL.');END IF;
END;
/
-- run result 
counter is NULL.

1.2 Procedure Prints BOOLEAN Variable"

DECLAREPROCEDURE print_x_and_y (x  BOOLEAN,y  BOOLEAN) ISBEGINprint_boolean ('x', x);print_boolean ('y', y);print_boolean ('x AND y', x AND y);END print_x_and_y;BEGINprint_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

DECLAREgrade CHAR(1); -- NULL by defaultappraisal VARCHAR2(20);
BEGINappraisal :=CASEWHEN 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.

BEGINprint_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
BEGINprint_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.

DECLAREPROCEDURE compare (value   VARCHAR2,pattern VARCHAR2) ISBEGINIF value LIKE pattern THENDBMS_OUTPUT.PUT_LINE ('TRUE');ELSEDBMS_OUTPUT.PUT_LINE ('FALSE');END IF;END;
BEGINcompare('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.

BEGINprint_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 = FALSEPL/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.

DECLAREletter VARCHAR2(1) := 'm';
BEGINprint_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') = TRUEPL/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

DECLAREa INTEGER; -- Initialized to NULL by defaultb INTEGER := 10;c INTEGER := 100;
BEGINprint_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) = NULLPL/SQL procedure successfully completed.