Добавил:
Опубликованный материал нарушает ваши авторские права? Сообщите нам.
Вуз: Предмет: Файл:
Скачиваний:
37
Добавлен:
16.04.2013
Размер:
5.97 Mб
Скачать

PL/SQL Expressions and Comparisons

LIKE Operator

You use the LIKE operator to compare a character, string, or CLOB value to a pattern. Case is significant. LIKE returns the Boolean value TRUE if the patterns match or FALSE if they do not match.

The patterns matched by LIKE can include two special-purpose characters called wildcards. An underscore (_) matches exactly one character; a percent sign (%) matches zero or more characters. For example, if the value of ename is 'JOHNSON', the following expression is true:

ename LIKE 'J%S_N'

To search for the percent sign and underscore characters, you define an escape character and put that character before the percent sign or underscore. The following example uses the backslash as the escape character, so that the percent sign in the string does not act as a wildcard:

IF sale_sign LIKE '50\% off!' ESCAPE '\' THEN...

BETWEEN Operator

The BETWEEN operator tests whether a value lies in a specified range. It means "greater than or equal to low value and less than or equal to high value." For example, the following expression is false:

45 BETWEEN 38 AND 44

IN Operator

The IN operator tests set membership. It means "equal to any member of." The set can contain nulls, but they are ignored. For example, the following expression tests whether a value is part of a set of values:

letter IN ('a','b','c')

Be careful when inverting this condition. Expressions of the form:

value NOT IN set

yield FALSE if the set contains a null.

Concatenation Operator

Double vertical bars (||) serve as the concatenation operator, which appends one string (CHAR, VARCHAR2, CLOB, or the equivalent Unicode-enabled type) to another. For example, the expression

'suit' || 'case'

returns the following value:

'suitcase'

If both operands have datatype CHAR, the concatenation operator returns a CHAR value. If either operand is a CLOB value, the operator returns a temporary CLOB. Otherwise, it returns a VARCHAR2 value.

Boolean Expressions

PL/SQL lets you compare variables and constants in both SQL and procedural statements. These comparisons, called Boolean expressions, consist of simple or complex

Fundamentals of the PL/SQL Language 2-21

PL/SQL Expressions and Comparisons

expressions separated by relational operators. Often, Boolean expressions are connected by the logical operators AND, OR, and NOT. A Boolean expression always yields TRUE, FALSE, or NULL.

In a SQL statement, Boolean expressions let you specify the rows in a table that are affected by the statement. In a procedural statement, Boolean expressions are the basis for conditional control. There are three kinds of Boolean expressions: arithmetic, character, and date.

Boolean Arithmetic Expressions

You can use the relational operators to compare numbers for equality or inequality. Comparisons are quantitative; that is, one number is greater than another if it represents a larger quantity. For example, given the assignments

number1 := 75; number2 := 70;

the following expression is true:

number1 > number2

Boolean Character Expressions

You can compare character values for equality or inequality. By default, comparisons are based on the binary values of each byte in the string.

For example, given the assignments

string1 := 'Kathy'; string2 := 'Kathleen';

the following expression is true:

string1 > string2

By setting the initialization parameter NLS_COMP=ANSI, you can make comparisons use the collating sequence identified by the NLS_SORT initialization parameter. A collating sequence is an internal ordering of the character set in which a range of numeric codes represents the individual characters. One character value is greater than another if its internal numeric value is larger. Each language might have different rules about where such characters occur in the collating sequence. For example, an accented letter might be sorted differently depending on the database character set, even though the binary value is the same in each case.

Depending on the value of the NLS_SORT parameter, you can perform comparisons that are case-insensitive and even accent-insensitive. A case-insensitive comparison still returns true if the letters of the operands are different in terms of uppercase and lowercase. An accent-insensitive comparison is case-insensitive, and also returns true if the operands differ in accents or punctuation characters. For example, the character values 'True' and 'TRUE' are considered identical by a case-insensitive comparison; the character values 'Cooperate', 'Co-Operate', and 'coöperate' are all considered the same. To make comparisons case-insensitive, add _CI to the end of your usual value for the NLS_SORT parameter. To make comparisons accent-insensitive, add _AI to the end of the NLS_SORT value.

There are semantic differences between the CHAR and VARCHAR2 base types that come into play when you compare character values. For more information, see Appendix B.

Many types can be converted to character types. For example, you can compare, assign, and do other character operations using CLOB variables. For details on the possible conversions, see "PL/SQL Character and String Types" on page 3-4.

2-22 PL/SQL User's Guide and Reference

PL/SQL Expressions and Comparisons

Boolean Date Expressions

You can also compare dates. Comparisons are chronological; that is, one date is greater than another if it is more recent. For example, given the assignments

date1 := '01-JAN-91'; date2 := '31-DEC-90';

the following expression is true:

date1 > date2

Guidelines for PL/SQL Boolean Expressions

In general, do not compare real numbers for exact equality or inequality. Real numbers are stored as approximate values. For example, the following IF condition might not yield TRUE:

DECLARE

fraction BINARY_FLOAT := 1/3; BEGIN

IF fraction = 11/33 THEN

dbms_output.put_line('Fractions are equal (luckily!)'); END IF;

END;

/

It is a good idea to use parentheses when doing comparisons. For example, the following expression is not allowed because 100 < tax yields a Boolean value, which cannot be compared with the number 500:

100 < tax < 500 -- not allowed

The debugged version follows:

(100 < tax) AND (tax < 500)

A Boolean variable is itself either true or false. You can just use the variable in a conditional test, rather than comparing it to the literal values TRUE and FALSE. For example, the following loops are all equivalent:

DECLARE

done BOOLEAN ; BEGIN

--Each WHILE loop is equivalent done := FALSE;

WHILE done = FALSE LOOP

done := TRUE; END LOOP;

done := FALSE;

WHILE NOT (done = TRUE) LOOP

done := TRUE; END LOOP;

done := FALSE; WHILE NOT done LOOP

done := TRUE;

END LOOP;

END;

Fundamentals of the PL/SQL Language 2-23

PL/SQL Expressions and Comparisons

/

Using CLOB values with comparison operators, or functions such as LIKE and BETWEEN, can create temporary LOBs. You might need to make sure your temporary tablespace is large enough to handle these temporary LOBs.

CASE Expressions

A CASE expression selects a result from one or more alternatives, and returns the result. Although it contains a block that might stretch over several lines, it really is an expression that forms part of a larger statement, such as an assignment or a procedure call.

The CASE expression uses a selector, an expression whose value determines which alternative to return. A CASE expression has the following form:

CASE selector

WHEN expression1 THEN result1 WHEN expression2 THEN result2

...

WHEN expressionN THEN resultN [ELSE resultN+1]

END

The selector is followed by one or more WHEN clauses, which are checked sequentially. The value of the selector determines which clause is evaluated. The first WHEN clause that matches the value of the selector determines the result value, and subsequent WHEN clauses are not evaluated. For example:

DECLARE

grade CHAR(1) := 'B'; appraisal VARCHAR2(20);

BEGIN appraisal :=

CASE grade

WHEN 'A' THEN 'Excellent'

WHEN 'B' THEN 'Very Good' WHEN 'C' THEN 'Good' WHEN 'D' THEN 'Fair' WHEN 'F' THEN 'Poor' ELSE 'No such grade'

END;

dbms_output.put_line('Grade ' || grade || ' is ' || appraisal); END;

/

The optional ELSE clause works similarly to the ELSE clause in an IF statement. If the value of the selector is not one of the choices covered by a WHEN clause, the ELSE clause is executed. If no ELSE clause is provided and none of the WHEN clauses are matched, the expression returns NULL.

An alternative to the CASE expression is the CASE statement, where each WHEN clause can be an entire PL/SQL block. For details, see "Using the CASE Statement" on

page 4-3.

Searched CASE Expression

PL/SQL also provides a searched CASE expression, which lets you test different conditions instead of comparing a single expression to various values. It has the form:

CASE

2-24 PL/SQL User's Guide and Reference

Тут вы можете оставить комментарий к выбранному абзацу или сообщить об ошибке.

Оставленные комментарии видны всем.

Соседние файлы в папке Oracle 10g