带有嵌套CASE和连接字符串的SQL给出尾随空格

Question

I am using Firebird 2.5. When I run

SELECT 
CASE BILANCA 
  WHEN 1 THEN
    CASE BILANCA WHEN 0 THEN 'SMALLINT' WHEN 1 THEN 'NUME' END
  ELSE 'A'||'B' END
  AS BILANCA1,
  CASE BILANCA 
  WHEN 1 THEN
    CASE BILANCA WHEN 0 THEN 'SMALLINT' WHEN 1 THEN 'NUME' END
  ELSE 'AB' END
  AS BILANCA2
FROM GS01BILANCA

field BILANCA2 is 'NUME ' (there are 4 spaces after NUME) and BILANCA1 is 'NUME' . Number of added spaces is equal to length of longest string in CASE. The only difference is in joining string in BILANCA1:

ELSE 'A'||'B' END

DDL is

CREATE TABLE GS01BILANCA
(BILANCA INTEGER DEFAULT  0 NOT NULL,
CONSTRAINT PK_GS01BILANCA PRIMARY KEY (BILANCA));

Table has 2 records:

• 1
• 2

Result is

• 'NUME ' (there are 4 spaces after NUME), 'NUME'

• 'AB' , 'AB'

Question: Is this expected and why?

Answer 1

String literals in Firebird are of type CHAR , the datatype of a case expression will have the longest length of all branches. So with

CASE BILANCA WHEN 0 THEN 'SMALLINT' WHEN 1 THEN 'NUME' END

The datatype will be CHAR(8) . A CHAR value will always be right-padded with the spaces to the declared length.

However when one of the branches of a case expression is a VARCHAR , then all branches are coerced to a VARCHAR . And concatenation in Firebird will always produce a VARCHAR , even if both operands are CHAR

This means that in the case of BILANCA1 :

CASE BILANCA 
WHEN 1 THEN
    CASE BILANCA WHEN 0 THEN 'SMALLINT' WHEN 1 THEN 'NUME' END
ELSE 'A'||'B' END

that 'A'||'B' is a VARCHAR(2) , this causes the entire case to be coerced to VARCHAR , which then causes 'SMALLINT' to become a VARCHAR(8) and 'NUME' a VARCHAR(4) , which makes the entire expression becomes a VARCHAR(8) .

Note that this type coercion is pushed down, so the literal will be a varchar from the start and not be blanked padded* by intermediate evaluation of the innermost case statement.

That is not the case for BILANCA2 , as all (nested) branches are literals, therefor it will be a CHAR(8) .

If you want both to behave the same you can do:

1. Cast the entire expression to the desired type (this would work both for bilanca1 and bilanca2 )
2. Cast part of the expression (eg one of the literals or the nested case) to a varchar ; this only works to force bilanca2 to a varchar
3. Cast the string concatenation to char ; this only works to force bilanca1 to a char
4. Concatenate an empty string to one of the literals; this only works to force bilanca2 to a varchar
5. Use trim on the entire expression (this also coerces to varchar )

---

*) This in contrast to for example cast(cast('nume' as char(8)) as varchar(8)) which will produce 'nume ' and not just 'nume' as in this case the intermediate result is preserved, and existing spaces are not thrown away in a varchar .