如何连接long nvarchar（max）行来构建动态T-SQL

Question

I have a table in SQL Server 2012 BI:

CREATE TABLE CodeParts (
    ID int identity(1,1) not null
    ,Line nvarchar(max) not null
)

loaded with parts of the very long T-SQL query stored in [Line] column. Example:

ID | Line
----------------------
 1 | BEGIN TRAN MERGE someTableWithLotOfColumns dst USING (SELECT...
 2 | WHEN MATCHED THEN CASE WHEN dst.someColumn != src.someColumn...
 3 | WHEN NOT MATCHED...
 4 | OUTPUT...
 5 | ;MERGE... next table with lot of columns blah blah blah
...| ...
25 | ;MERGE... yet another table with lot of columns
60 | COMMIT

The code have 60 lines, each of the line may be up to 12,000 characters because of number of the columns and their names' length.

I need to execute entire code built by all those rows and I don't know how to do that avoiding truncation.

Answer 1

It can be very tricky to work with longer string. Check this:

DECLARE @txt NVARCHAR(MAX)=(SELECT REPLICATE('x',12000));
SELECT LEN(@txt) AS CountCharacters
      ,DATALENGTH(@txt) AS UsedBytes;

Although one might think this is declared as NVARCHAR(MAX) the given 'x' isn't. This let's the string be a normal string with a smaller size limit. Now try this (only difference is the CAST('x' AS NVARCHAR(MAX)) ):

DECLARE @txt2 NVARCHAR(MAX)=(SELECT REPLICATE(CAST('x' AS NVARCHAR(MAX)),12000));
SELECT LEN(@txt2) AS CountCharacters
      ,DATALENGTH(@txt2) AS UsedBytes;

To demonstrate this I create a working example with a dummy table with 60 row each row consisting of 12.000 characters.

DECLARE @tbl TABLE(ID INT IDENTITY,CodeLine NVARCHAR(MAX));

WITH TallySixty AS (SELECT TOP 60 ROW_NUMBER() OVER(ORDER BY (SELECT NULL)) AS Dummy FROM master..spt_values)
INSERT INTO @tbl
SELECT REPLICATE(CAST(RIGHT(Dummy,1) AS NVARCHAR(MAX)),12000)
FROM TallySixty;

SELECT CodeLine,LEN(CodeLine) AS CountCharacters
      ,DATALENGTH(CodeLine) AS UsedBytes FROM @tbl

DECLARE @concatString NVARCHAR(MAX)=
(
    SELECT(SELECT CodeLine + ' '  FROM @tbl FOR XML PATH(''),TYPE).value('(text())[1]','nvarchar(max)')
);

SELECT @concatString
      ,LEN(@concatString) AS CountCharacters
      ,DATALENGTH(@concatString) AS UsedBytes

The final result shows clearly, that the resulting string has the length of 60 times 12.000 (plus the added blanks) and is twice this sice in memory due to NVARCHAR . Up to ~2GB this concatenation should work. According to this this is pretty much enough :-)

I think, that EXEC is able to deal with NVARCHAR(MAX) up to the full size.

Answer 2

DECLARE @sql NVARCHAR(max)
SET @sql=ISNULL(@sql+CHAR(13),'')+Line FROM CodeParts order by id
EXEC(@SQL)

The @sql variable must be declared with MAX as length. If the string is more than 4000, you may cannot print whole string, but it can be executed.