SQL 케이스 표현식 구문?

SQL 케이스 표현식 구문?

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SQL Case 표현식에 대한 완전 하고 올바른 구문 은 무엇입니까 ?

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전체 구문은 작업중인 데이터베이스 엔진에 따라 달라집니다

SQL Server의 경우 :

CASE case-expression
    WHEN when-expression-1 THEN value-1
  [ WHEN when-expression-n THEN value-n ... ]
  [ ELSE else-value ]
END

또는:

CASE
    WHEN boolean-when-expression-1 THEN value-1
  [ WHEN boolean-when-expression-n THEN value-n ... ]
  [ ELSE else-value ]
END

식 등 :

case-expression    - something that produces a value
when-expression-x  - something that is compared against the case-expression
value-1            - the result of the CASE statement if:
                         the when-expression == case-expression
                      OR the boolean-when-expression == TRUE
boolean-when-exp.. - something that produces a TRUE/FALSE answer

링크 : CASE (Transact-SQL)

또한 WHEN 문의 순서도 중요합니다. 겹치는 여러 WHEN 절을 쉽게 작성할 수 있으며 일치하는 첫 번째 절이 사용됩니다 .

참고 : ELSE 절이 지정되지 않고 일치하는 WHEN-condition이없는 경우 CASE 표현식의 값은 NULL 입니다.

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여러 제품에 태그를 지정한 것을 고려할 때 ISO / ANSI SQL-92 표준에서 찾을 수있는 완전한 구문은 다음과 같습니다.

<case expression> ::=
       <case abbreviation>
     | <case specification>

<case abbreviation> ::=
       NULLIF <left paren> <value expression> <comma>
              <value expression> <right paren>
     | COALESCE <left paren> <value expression>
                { <comma> <value expression> }... <right paren>

<case specification> ::=
       <simple case>
     | <searched case>

<simple case> ::=
     CASE <case operand>
          <simple when clause>...
        [ <else clause> ]
     END

<searched case> ::=
     CASE
       <searched when clause>...
     [ <else clause> ]
     END

<simple when clause> ::= WHEN <when operand> THEN <result>

<searched when clause> ::= WHEN <search condition> THEN <result>

<else clause> ::= ELSE <result>

<case operand> ::= <value expression>

<when operand> ::= <value expression>

<result> ::= <result expression> | NULL

<result expression> ::= <value expression>

구문 규칙

1) NULLIF (V1, V2) is equivalent to the following <case specification>:

     CASE WHEN V1=V2 THEN NULL ELSE V1 END

2) COALESCE (V1, V2) is equivalent to the following <case specification>:

     CASE WHEN V1 IS NOT NULL THEN V1 ELSE V2 END

3) COALESCE (V1, V2, . . . ,n ), for n >= 3, is equivalent to the
   following <case specification>:

     CASE WHEN V1 IS NOT NULL THEN V1 ELSE COALESCE (V2, . . . ,n )
     END

4) If a <case specification> specifies a <simple case>, then let CO
   be the <case operand>:

   a) The data type of each <when operand> WO shall be comparable
      with the data type of the <case operand>.

   b) The <case specification> is equivalent to a <searched case>
      in which each <searched when clause> specifies a <search
      condition> of the form "CO=WO".

5) At least one <result> in a <case specification> shall specify a
   <result expression>.

6) If an <else clause> is not specified, then ELSE NULL is im-
   plicit.

7) The data type of a <case specification> is determined by ap-
   plying Subclause 9.3, "Set operation result data types", to the
   data types of all <result expression>s in the <case specifica-
   tion>.

Access Rules

   None.

General Rules

1) Case:

   a) If a <result> specifies NULL, then its value is the null
      value.

   b) If a <result> specifies a <value expression>, then its value
      is the value of that <value expression>.

2) Case:

   a) If the <search condition> of some <searched when clause> in
      a <case specification> is true, then the value of the <case
      specification> is the value of the <result> of the first
      (leftmost) <searched when clause> whose <search condition> is
      true, cast as the data type of the <case specification>.

   b) If no <search condition> in a <case specification> is true,
      then the value of the <case expression> is the value of the
      <result> of the explicit or implicit <else clause>, cast as
      the data type of the <case specification>.

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Here are the CASE statement examples from the PostgreSQL docs (Postgres follows the SQL standard here):

SELECT a,
   CASE WHEN a=1 THEN 'one'
        WHEN a=2 THEN 'two'
        ELSE 'other'
   END
FROM test;

or

SELECT a,
   CASE a WHEN 1 THEN 'one'
          WHEN 2 THEN 'two'
          ELSE 'other'
   END
FROM test;

Obviously the second form is cleaner when you are just checking one field against a list of possible values. The first form allows more complicated expressions.

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Sybase has the same case syntax as SQL Server:

Description

Supports conditional SQL expressions; can be used anywhere a value expression can be used.

Syntax

case 
     when search_condition then expression 
    [when search_condition then expression]...
    [else expression]
end

Case and values syntax

case expression
     when expression then expression 
    [when expression then expression]...
    [else expression]
end

Parameters

case

begins the case expression.

when

precedes the search condition or the expression to be compared.

search_condition

is used to set conditions for the results that are selected. Search conditions for case expressions are similar to the search conditions in a where clause. Search conditions are detailed in the Transact-SQL User’s Guide.

then

precedes the expression that specifies a result value of case.

expression

is a column name, a constant, a function, a subquery, or any combination of column names, constants, and functions connected by arithmetic or bitwise operators. For more information about expressions, see “Expressions” in.

Example

select disaster, 
       case
            when disaster = "earthquake" 
                then "stand in doorway"
            when disaster = "nuclear apocalypse" 
                then "hide in basement"
            when monster = "zombie apocalypse" 
                then "hide with Chuck Norris"
            else
                then "ask mom"
       end 
  from endoftheworld

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I dug up the Oracle page for the same and it looks like this is the same syntax, just described slightly different.

Link: Oracle/PLSQL: Case Statement

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Oracle syntax from the 11g Documentation:

CASE { simple_case_expression | searched_case_expression }
     [ else_clause ]
     END

simple_case_expression

expr { WHEN comparison_expr THEN return_expr }...

searched_case_expression

{ WHEN condition THEN return_expr }...

else_clause

ELSE else_expr

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Case statement syntax in SQL SERVER:

CASE column
   WHEN value1 THEN 1
   WHEN value3 THEN 2
   WHEN value3 THEN 3
   WHEN value1 THEN 4
   ELSE ''
END

And we can use like below also:

CASE 
   WHEN column=value1 THEN 1
   WHEN column=value3 THEN 2
   WHEN column=value3 THEN 3
   WHEN column=value1 THEN 4
   ELSE ''
END

참고URL : https://stackoverflow.com/questions/4622/sql-case-expression-syntax