SQL Server: how to know if any row is referencing the row to delete

Question

You cannot delete a row if any row is referencing the row to delete via a FK.

Is it possible to know if any row is referencing the row to delete before executing a DELETE statement?

Answer 1

This script will show all the tables that have rows that reference the row you are trying to delete:

declare @RowId int = 1
declare @TableName sysname = 'ParentTable'

declare @Command varchar(max) 

select @Command = isnull(@Command + ' union all ', '') + 'select ''' + object_name(parent_object_id) + 
    ''' where exists(select * from ' + object_name(parent_object_id) + ' where ' + col.name+ ' = ' + cast(@RowId as varchar) + ')' 
from sys.foreign_key_columns fkc
    join sys.columns col on
        fkc.parent_object_id = col.object_id and fkc.parent_column_id = col.column_id
where object_name(referenced_object_id) = @TableName

execute (@Command)

Assumption that foreign key is not composite.

Answer 2

Option 1 (Detection):

You perform a Select Statement to determine if any records are referencing the Record-to-be-deleted -- and, if you wish, manually delete those records that do reference it. This can also be accomplished using a trigger, although I recommend against triggers, because they tend to surprise people (and yourself) down the road.

Option 2 (Automation):

You can look into Cascading Deletes which, if configured correctly, will cause all records referencing the Record-to-be-deleted to also be deleted.

When to use Cascading Deletes (Paraphrased from text written by Joel Coehoorn)

• Cascade Delete may make sense when the semantics of the relationship can involve an "is part of" description. Example: Web Order, Web Order Line Items
• You should not use Cascade Delete if you are preserving history or using a soft delete where you only set a deleted bit column
• Cascading can get you into trouble if you set up your foreign keys wrong.
• It's not wise to use cascading before you understand it thoroughly. However, it is a useful feature and therefore worth taking the time to understand.

Here's a great discussion on Cascading Deletes on stackoverflow.

Answer 3

nobody has mentioned it, but just for the record I use a lot the procedure

sp_helpconstraint 'dbo.mytable'

in order to find all the constraints related to dbo.mytable, and which tables reference dbo.mytable. I find it very useful and handy.

Answer 4

I improved the Alex Aza's solution.

I use softdelete, so It's necessary add a column "delete" in the condition "where" . And more I made a function in TSQL that it discovers when the table represents the object inherited in NHibernate, and the PK is the FK from parent table.

Follow:

declare @RowId int = 4
declare @TableName sysname = 'TABLE'

declare @Command varchar(max) 

select @Command = isnull(@Command + ' union all ', '') + 'select ''' + object_name(parent_object_id) + 
    ''' where exists(select * from ' + object_name(parent_object_id) + 
    CASE
        WHEN EXISTS(select object_name(object_id) from sys.columns col where name = 'deleted' and object_id = parent_object_id) 
            THEN ' where ' + col.name+ ' = ' + cast(@RowId as varchar) +' and deleted = 0 '
        when dbo.ParentIdFromTable(object_name(parent_object_id)) <> ''
            then ' inner join ' + dbo.ParentIdFromTable(object_name(parent_object_id)) + ' on id = ' + dbo.PrimaryKey(object_name(parent_object_id))
                +' where ' + col.name+ ' = ' + cast(@RowId as varchar) +' and deleted = 0 '
        else 
            ' where ' + col.name+ ' = ' + cast(@RowId as varchar) 
      END
    + ')' 
from sys.foreign_key_columns fkc
    join sys.columns col on
        fkc.parent_object_id = col.object_id and fkc.parent_column_id = col.column_id
where object_name(referenced_object_id) = @TableName

PRINT @Command
execute (@Command)

Depedencies Functions:

CREATE FUNCTION dbo.ParentIdFromTable(@Table varchar(255))
RETURNS varchar(255) 
AS 
BEGIN
    declare @tableParent varchar(255) = ''

    if exists(select pk.TABLE_NAME, pk.COLUMN_NAME, col.name, object_name(referenced_object_id) Referenced, object_name(parent_object_id) as Parent
        from sys.columns col
            inner join sys.foreign_key_columns fkc on
                fkc.parent_object_id = col.object_id and fkc.parent_column_id = col.column_id
            inner join INFORMATION_SCHEMA.KEY_COLUMN_USAGE pk on
                pk.TABLE_NAME = object_name(object_id) and pk.COLUMN_NAME = col.name
        WHERE OBJECTPROPERTY(OBJECT_ID(constraint_name), 'IsPrimaryKey') = 1
        AND table_name = @table)
    begin

        while exists(select *
            from sys.columns col
            inner join sys.foreign_key_columns fkc on
                fkc.parent_object_id = col.object_id and fkc.parent_column_id = col.column_id
            inner join INFORMATION_SCHEMA.KEY_COLUMN_USAGE pk on
                pk.TABLE_NAME = object_name(object_id) and pk.COLUMN_NAME = col.name
            WHERE OBJECTPROPERTY(OBJECT_ID(constraint_name), 'IsPrimaryKey') = 1 AND table_name = @table)
        begin
            -- Descobrir o parent, column
            select  @tableParent = object_name(referenced_object_id)
            from sys.columns col
            inner join sys.foreign_key_columns fkc on
                fkc.parent_object_id = col.object_id and fkc.parent_column_id = col.column_id
            inner join INFORMATION_SCHEMA.KEY_COLUMN_USAGE pk on
                pk.TABLE_NAME = object_name(object_id) and pk.COLUMN_NAME = col.name
            WHERE OBJECTPROPERTY(OBJECT_ID(constraint_name), 'IsPrimaryKey') = 1
            AND table_name = @table

            --print @tableParent
            set @table = @tableParent
        end
    end

    return @tableParent;
END;
GO


CREATE FUNCTION dbo.PrimaryKey(@Table varchar(255))
RETURNS varchar(255) 
AS 
BEGIN
    declare @columnName varchar(255) = ''
    -- Descobrir o parent, column
    select @columnName = COLUMN_NAME
    from INFORMATION_SCHEMA.KEY_COLUMN_USAGE 
    WHERE OBJECTPROPERTY(OBJECT_ID(constraint_name), 'IsPrimaryKey') = 1
    AND table_name = @Table

    return @columnName
end;