ORDER BY changes String Concatenation SQL

Question

I was trying to build a script that would drop all foreign keys in a particular schema (I'm working on creating a new database which will need to be scripted to several other locations, a test environment and a production one). The easiest way to have these three separate environments is to work with my "CREATE" script on "development", and then stage that through our other environments at a later date. This requires (as one possible solution) that the beginning of the "CREATE" script is a sort of If Exists(...) DROP....

I have solved the problem I was having, but was hoping that someone could explain the behavior I was seeing (see below).

This code works (I get a statement that has all records as their own SQL statements):

DECLARE @SQL VARCHAR(MAX) = ''
DECLARE @Schema VARCHAR(100) = 'SchemaName'
SELECT 
    @SQL = @SQL
         + 'ALTER TABLE ' + tpa.name + CHAR(13) + CHAR(10)
         + CHAR(9) + 'DROP CONSTRAINT ' + fk.name +';' + CHAR(13) + CHAR(10)
FROM 
    sys.foreign_keys AS fk
JOIN sys.foreign_key_columns AS fkc 
    ON fkc.constraint_object_id = fk.object_id
JOIN sys.tables AS tpa
    ON fk.parent_object_id = tpa.object_id
JOIN sys.columns AS cpa 
    ON tpa.object_id = cpa.object_id AND 
       fkc.parent_column_id = cpa.column_id
JOIN sys.tables AS tref
    ON fkc.parent_object_id = tref.object_id
JOIN sys.columns AS cref 
    ON tref.object_id = cref.object_id AND 
       fkc.referenced_column_id = cref.column_id
WHERE 
    SCHEMA_NAME(fk.schema_id) = @Schema

PRINT @SQL 

This code doesn't work (it only returns the last line of the recordset):

DECLARE @SQL VARCHAR(MAX) = ''
DECLARE @Schema VARCHAR(100) = 'SchemaName'
SELECT 
    @SQL = @SQL
         + 'ALTER TABLE ' + tpa.name + CHAR(13) + CHAR(10)
         + CHAR(9) + 'DROP CONSTRAINT ' + fk.name +';' + CHAR(13) + CHAR(10)
FROM 
    sys.foreign_keys AS fk
JOIN sys.foreign_key_columns AS fkc 
    ON fkc.constraint_object_id = fk.object_id
JOIN sys.tables AS tpa
    ON fk.parent_object_id = tpa.object_id
JOIN sys.columns AS cpa 
    ON tpa.object_id = cpa.object_id AND 
       fkc.parent_column_id = cpa.column_id
JOIN sys.tables AS tref
    ON fkc.parent_object_id = tref.object_id
JOIN sys.columns AS cref 
    ON tref.object_id = cref.object_id AND 
       fkc.referenced_column_id = cref.column_id
WHERE 
    SCHEMA_NAME(fk.schema_id) = @Schema
ORDER BY 
     OBJECT_NAME(fk.parent_object_id)
    ,OBJECT_NAME(fk.referenced_object_id)
    ,fk.name
PRINT @SQL

The offending line seems to be the OBJECT_NAME(fk.referenced_object_id) in the ORDER BY as commenting that out resolves the issue, but I don't understand why it's failing. My suspicion is that something is doing a group by inherently or something.

Even including all columns from the ORDER BY doesn't seem to help:

DECLARE @SQL VARCHAR(MAX) = ''
DECLARE @Schema VARCHAR(100) = 'UserMgmt'
SELECT 
    @SQL = @SQL
         + 'ALTER TABLE ' + tpa.name + CHAR(13) + CHAR(10)
         + CHAR(9) + 'DROP CONSTRAINT ' + fk.name +';' + CHAR(13) + CHAR(10)
         + '--' + OBJECT_NAME(fk.referenced_object_id) + ', ' + OBJECT_NAME(fk.parent_object_id) + CHAR(13) + CHAR(10)
FROM 
    sys.foreign_keys AS fk
JOIN sys.foreign_key_columns AS fkc 
    ON fkc.constraint_object_id = fk.object_id
JOIN sys.tables AS tpa
    ON fk.parent_object_id = tpa.object_id
JOIN sys.columns AS cpa 
    ON tpa.object_id = cpa.object_id AND 
       fkc.parent_column_id = cpa.column_id
JOIN sys.tables AS tref
    ON fkc.parent_object_id = tref.object_id
JOIN sys.columns AS cref 
    ON tref.object_id = cref.object_id AND 
       fkc.referenced_column_id = cref.column_id
WHERE 
    SCHEMA_NAME(fk.schema_id) = @Schema
ORDER BY 
     OBJECT_NAME(fk.parent_object_id)
    ,OBJECT_NAME(fk.referenced_object_id)
    ,fk.name
PRINT @SQL

However, changing the JOIN statement for the referenced table does make this work:

DECLARE @SQL VARCHAR(MAX) = ''
DECLARE @Schema VARCHAR(100) = 'UserMgmt'
SELECT 
    @SQL = @SQL
         + 'ALTER TABLE ' + tpa.name + CHAR(13) + CHAR(10)
         + CHAR(9) + 'DROP CONSTRAINT ' + fk.name +';' + CHAR(13) + CHAR(10)
         + '--' + OBJECT_NAME(fk.referenced_object_id) + ', ' + OBJECT_NAME(fk.parent_object_id) + CHAR(13) + CHAR(10)
FROM 
    sys.foreign_keys AS fk
JOIN sys.foreign_key_columns AS fkc 
    ON fkc.constraint_object_id = fk.object_id
JOIN sys.tables AS tpa
    ON fk.parent_object_id = tpa.object_id
JOIN sys.columns AS cpa 
    ON tpa.object_id = cpa.object_id AND 
       fkc.parent_column_id = cpa.column_id
JOIN sys.tables AS tref
    ON fk.referenced_object_id = tref.object_id
JOIN sys.columns AS cref 
    ON tref.object_id = cref.object_id AND 
       fkc.referenced_column_id = cref.column_id
WHERE 
    SCHEMA_NAME(fk.schema_id) = @Schema
ORDER BY 
     OBJECT_NAME(fk.parent_object_id)
    ,OBJECT_NAME(fk.referenced_object_id)
    ,fk.name
PRINT @SQL

Can anyone explain this behavior?

Answer 1

The reason is as Martin Smith pointed out: there is no defined behavior with aggregated string concatenation.

The solution is to use a cursor instead of pretending you're not. You should also (a) not use VARCHAR for metadata - it's all stored as Unicode so you should use SYSNAME/NVARCHAR (b) use QUOTENAME around entity names in case they are reserved words or otherwise invalid identifiers (c) obtain the ID for the schema once instead of inline.

DECLARE @schemaID INT = SCHEMA_ID(N'SchemaName');

DECLARE @sql NVARCHAR(MAX) = N'', @tpaname SYSNAME, @fkname SYSNAME;

DECLARE c CURSOR LOCAL FAST_FORWARD FOR
SELECT tpa.name, fk.name FROM sys.foreign_keys AS fk
INNER JOIN sys.foreign_key_columns AS fkc ON fkc.constraint_object_id = fk.object_id
INNER JOIN sys.tables AS tpa ON fk.parent_object_id = tpa.object_id
INNER JOIN sys.columns AS cpa ON tpa.object_id = cpa.object_id AND 
       fkc.parent_column_id = cpa.column_id
INNER JOIN sys.tables AS tref ON fkc.parent_object_id = tref.object_id
INNER JOIN sys.columns AS cref ON tref.object_id = cref.object_id AND 
       fkc.referenced_column_id = cref.column_id
WHERE fk.schema_id = @SchemaID
ORDER BY 
     OBJECT_NAME(fk.parent_object_id)
    ,OBJECT_NAME(fk.referenced_object_id)
    ,fk.name;

OPEN c;

FETCH c INTO @tpaname, @fkname;

WHILE @@FETCH_STATUS <> -1;
BEGIN
  SET @sql += N'ALTER TABLE ' + QUOTENAME(@tpaname) + CHAR(13) + CHAR(10)
         + CHAR(9) + 'DROP CONSTRAINT ' + QUOTENAME(@fkname) +';' + CHAR(13) + CHAR(10)
  FETCH c INTO @tpaname, @fkname;
END

CLOSE c; DEALLOCATE c;

PRINT @sql;