Oracle CASE Expressions

CASE Expressions
Topics

• Simple CASE Expression
• Searched CASE Expression

1. Simple CASE Expression

For this explanation, assume that a simple CASE expression has this syntax:

CASE selector
WHEN { selector_value_1a | dangling_predicate_1a }
  [ , ..., { selector_value_1n | dangling_predicate_1n } ] THEN result_1
WHEN { selector_value_2a | dangling_predicate_2a }
  [ , ..., { selector_value_2n | dangling_predicate_2n } ] THEN result_2
...
WHEN { selector_value_na | dangling_predicate_na }
  [ , ..., { selector_value_nn | dangling_predicate_nn }] THEN result_n
[ ELSE
  else_result ]
END;

The selector is an expression (typically a single variable). Each selector_value and each result can be either a literal or an expression. A dangling_predicate can also be used either instead of or in combination with one or multiple selector_values. At least one result must not be the literal NULL.

A dangling_predicate is an ordinary expression with its left operand missing, for example < 2. Using a dangling_predicate allows for more complicated comparisons that would otherwise require a searched CASE statement.

The simple CASE expression returns the first result for which the selector_value or dangling_predicate matches selector. Remaining expressions are not evaluated. If no selector_value or dangling_predicate matches selector, the CASE expression returns else_result if it exists and NULL otherwise.

A list of comma-separated selector_values and or dangling_predicates can be used with each WHEN clause if multiple choices map to a single result. As with selector_values and dangling_predicates listed in separate WHEN clauses, only the first selector_value or dangling_predicate to match the selector is evaluated.

1.1 Simple CASE Expression

This example assigns the value of a simple CASE expression to the variable appraisal. The selector is grade.

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;
/
-- run result
Grade B is Very Good
 

1.2 Simple CASE Expression with WHEN NULL

If selector has the value NULL, it cannot be matched by WHEN NULL, as this example shows.

Instead, use a searched CASE expression with WHEN boolean_expression IS NULL

DECLARE
  grade CHAR(1); -- NULL by default
  appraisal VARCHAR2(20);
BEGIN
  appraisal :=
  CASE grade
    WHEN NULL THEN 'No grade assigned'
    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;
/
-- run result
Grade  is No such grade

2.3 Simple CASE Expression with List of selector_values

oracle 23ai 新特性，oracle19c及以下版本不支持

DECLARE
    salary NUMBER := 7000;
    salary_level VARCHAR2(20);
BEGIN
    salary_level :=
CASE salary
WHEN 1000, 2000 THEN 'low'
WHEN 3000, 4000, 5000 THEN 'normal'
WHEN 6000, 7000, 8000 THEN 'high'
ELSE 'executive pay'
END;
DBMS_OUTPUT.PUT_LINE('Salary level is: ' || salary_level);
END;
/
-- run result
Salary level is: high

PL/SQL procedure successfully completed.

2.4 Simple CASE Expression with Dangling Predicates

oracle 23ai 新特性，oracle19c及以下版本不支持
The value of data_val/2 is used as the left operand during evaluation of the dangling_predicates. Using a simple CASE expression as opposed to a searched CASE expression in this situation avoids repeated computation of the selector expression. You can use a list of conditions with any combination of selector_values and dangling_predicates.

DECLARE
    data_val NUMBER := 30;
    status VARCHAR2(20);
BEGIN
    status :=
CASE data_val/2
WHEN < 0, > 50 THEN 'outlier'
WHEN BETWEEN 10 AND 30 THEN 'good'
ELSE 'bad'
END;
DBMS_OUTPUT.PUT_LINE('The data status is: ' || status);
END;
/
-- run reslut
The data status is: good

PL/SQL procedure successfully completed.

2. Searched CASE Expression

For this explanation, assume that a searched CASE expression has this syntax:

CASE
WHEN boolean_expression_1 THEN result_1
WHEN boolean_expression_2 THEN result_2
...
WHEN boolean_expression_n THEN result_n
[ ELSE
  else_result ]
END]

2.1 Searched CASE Expression

This example assigns the value of a searched CASE expression to the variable appraisal.

DECLARE
  grade      CHAR(1) := 'B';
  appraisal  VARCHAR2(120);
  id         NUMBER  := 8429862;
  attendance NUMBER := 150;
  min_days   CONSTANT NUMBER := 200;
  
  FUNCTION attends_this_school (id NUMBER)
    RETURN BOOLEAN IS
  BEGIN
    RETURN TRUE;
  END;
BEGIN
  appraisal :=
  CASE
    WHEN attends_this_school(id) = FALSE
      THEN 'Student not enrolled'
    WHEN grade = 'F' OR attendance < min_days
      THEN 'Poor (poor performance or bad attendance)'
    WHEN grade = 'A' THEN 'Excellent'
    WHEN grade = 'B' THEN 'Very Good'
    WHEN grade = 'C' THEN 'Good'
    WHEN grade = 'D' THEN 'Fair'
    ELSE 'No such grade'
  END;
  DBMS_OUTPUT.PUT_LINE
    ('Result for student ' || id || ' is ' || appraisal);
END;
/
-- run result 
Result for student 8429862 is Poor (poor performance or bad attendance)
 

2.2 Searched CASE Expression with WHEN … IS NULL

This example uses a searched CASE expression

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

PL/SQL procedure successfully completed. 

原文地址：https://blog.csdn.net/zxrhhm/article/details/142499829

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