How to identify one-to-one (one-to-?) relationship using INFORMATION_SCHEMA or sys views in Sql Server

Question

So here is the problem.. Using TSQL and INFORMATION_SCHEMA or sys views, how can I identify a 1:0-1 relationship, such as FK_BaseTable_InheritedTable?

In a concrete example, imagine a simple DTO - FK_JoinTable_ParentTable would be rendered as a collection of JoinTable on the ParentTable object, whereas FK_BaseTable_InheritedTable would either be rendered as an InheritedTable object on the BaseTable object (inheritance was a bad choice for example, I know, but not going back).

The best I can come up with is one-to-many, same as FK_JoinTable_ParentTable. I have tried a lot of approaches, including (trying to) comparing keys and am coming up short.

Here is the script. Problem is to, USING INFO_SCHEMA or sys views , identify FK_JoinTable_ParentTable and FK_JoinTable_Child as one-to-many and FK_BaseTable_InheritedTable as one-to-one/none.

The litmus is being able to differentiate FK_BaseTable_InheritedTable from FK_JoinTable_ParentTable

CREATE TABLE [dbo].[Child](
 [ChildId] [int] NOT NULL,
 CONSTRAINT [PK_Child] PRIMARY KEY CLUSTERED 
(
 [ChildId] ASC
)WITH (PAD_INDEX  = OFF, STATISTICS_NORECOMPUTE  = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS  = ON, ALLOW_PAGE_LOCKS  = ON) ON [PRIMARY]
) ON [PRIMARY]

CREATE TABLE [dbo].[ParentTable](
 [ParentId] [int] NOT NULL,
 CONSTRAINT [PK_ParentTable] PRIMARY KEY CLUSTERED 
(
 [ParentId] ASC
)WITH (PAD_INDEX  = OFF, STATISTICS_NORECOMPUTE  = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS  = ON, ALLOW_PAGE_LOCKS  = ON) ON [PRIMARY]
) ON [PRIMARY]


CREATE TABLE [dbo].[JoinTable](
 [PId] [int] NOT NULL,
 [CId] [int] NOT NULL,
 CONSTRAINT [PK_JoinTable] PRIMARY KEY CLUSTERED 
(
 [PId] ASC,
 [CId] ASC
)WITH (PAD_INDEX  = OFF, STATISTICS_NORECOMPUTE  = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS  = ON, ALLOW_PAGE_LOCKS  = ON) ON [PRIMARY]
) ON [PRIMARY]

CREATE TABLE [dbo].[InheritedTable](
 [InheritedId] [int] NOT NULL,
 CONSTRAINT [PK_InheritedTable] PRIMARY KEY CLUSTERED 
(
 [InheritedId] ASC
)WITH (PAD_INDEX  = OFF, STATISTICS_NORECOMPUTE  = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS  = ON, ALLOW_PAGE_LOCKS  = ON) ON [PRIMARY]
) ON [PRIMARY]

CREATE TABLE [dbo].[BaseTable](
 [BaseId] [int] NOT NULL,
 CONSTRAINT [PK_BaseTable] PRIMARY KEY CLUSTERED 
(
 [BaseId] ASC
)WITH (PAD_INDEX  = OFF, STATISTICS_NORECOMPUTE  = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS  = ON, ALLOW_PAGE_LOCKS  = ON) ON [PRIMARY]
) ON [PRIMARY]

ALTER TABLE [dbo].[JoinTable]  WITH CHECK ADD  CONSTRAINT [FK_JoinTable_Child] FOREIGN KEY([CId])
REFERENCES [dbo].[Child] ([ChildId])

ALTER TABLE [dbo].[JoinTable] CHECK CONSTRAINT [FK_JoinTable_Child]

ALTER TABLE [dbo].[JoinTable]  WITH CHECK ADD  CONSTRAINT [FK_JoinTable_ParentTable] FOREIGN KEY([PId])
REFERENCES [dbo].[ParentTable] ([ParentId])

ALTER TABLE [dbo].[JoinTable] CHECK CONSTRAINT [FK_JoinTable_ParentTable]

ALTER TABLE [dbo].[BaseTable]  WITH CHECK ADD  CONSTRAINT [FK_BaseTable_InheritedTable] FOREIGN KEY([BaseId])
REFERENCES [dbo].[InheritedTable] ([InheritedId])

ALTER TABLE [dbo].[BaseTable] CHECK CONSTRAINT [FK_BaseTable_InheritedTable]

Answer 1

EDIT: fixed small typo in the foreign key query that does not affect the answer but returns the wrong unique table

The original conclusion I came to but was never able to properly execute is:

If the base side columns of the constraint fit into any one of the base side table's unique constraints AND the column count is the same then it is a one-to-one/none.

The same could be said if the all of the FK columns fit one of the unique constraints which has more columns IF all of those columns comprise yet another FK, but this is outside the scope of the question at hand.

It was an execution issue on my part.

I worked out a solution that produces correct results but in that I am no SQL guru I fear it is rather kludgy. Perhaps I will put it up as another question for review.

Anyway - this script will identify all the 1:? relationships in the db.

/*
    Will identify immediate 1:? fk relationships

*/
-- TODO: puzzle: work out the set-based equivalent 

SET NOCOUNT ON




BEGIN -- Get a table full of PK and UQ columns
    DECLARE @unique_keys TABLE
        (
          -- contains PK and UQ indexes
          [schema_name] NVARCHAR(128),
          table_name NVARCHAR(128),
          index_name NVARCHAR(128),
          column_id INT,
          column_name NVARCHAR(128),
          is_primary_key BIT,
          is_unique_constraint BIT,
          is_unique BIT
        )
    INSERT  INTO @unique_keys
            (
              [schema_name],
              table_name,
              index_name,
              column_id,
              column_name,
              is_primary_key,
              is_unique_constraint,
              is_unique
            )
        -- selects PK and UQ indexes
            SELECT  S.name AS [schema_name],
                    T.name AS table_name,
                    IX.name AS index_name,
                    IC.column_id,
                    C.name AS column_name,
                    IX.is_primary_key,
                    IX.is_unique_constraint,
                    IX.is_unique
            FROM    sys.tables AS T
                    INNER JOIN sys.schemas AS S ON T.schema_id = S.schema_id
                    INNER JOIN sys.indexes AS IX ON T.object_id = IX.object_id
                    INNER JOIN sys.index_columns AS IC ON IX.object_id = IC.object_id
                                                          AND IX.index_id = IC.index_id
                    INNER JOIN sys.columns AS C ON IC.column_id = C.column_id
                                                   AND IC.object_id = C.object_id
            WHERE   ( IX.is_unique = 1 )
                    AND ( T.name <> 'sysdiagrams' )
                    AND IX.is_unique = 1
            ORDER BY schema_name,
                    table_name,
                    index_name,
                    C.column_id
END



BEGIN -- Get a table full of FK columns

    DECLARE @foreign_key_columns TABLE
        (
          constraint_name NVARCHAR(128),
          base_schema_name NVARCHAR(128),
          base_table_name NVARCHAR(128),
          base_column_id INT,
          base_column_name NVARCHAR(128),
          unique_schema_name NVARCHAR(128),
          unique_table_name NVARCHAR(128),
          unique_column_id INT,
          unique_column_name NVARCHAR(128)
        )
    INSERT  INTO @foreign_key_columns
            (
              constraint_name,
              base_schema_name,
              base_table_name,
              base_column_id,
              base_column_name,
              unique_schema_name,
              unique_table_name,
              unique_column_id,
              unique_column_name
            )
            SELECT  FK.name AS constraint_name,
                    S.name AS base_schema_name,
                    T.name AS base_table_name,
                    C.column_id AS base_column_id,
                    C.name AS base_column_name,
                    US.name AS unique_schema_name,
                    UT.name AS unique_table_name,
                    UC.column_id AS unique_column_id,
                    UC.name AS unique_column_name
            FROM    sys.tables AS T
                    INNER JOIN sys.schemas AS S ON T.schema_id = S.schema_id
                    INNER JOIN sys.foreign_keys AS FK ON T.object_id = FK.parent_object_id
                    INNER JOIN sys.foreign_key_columns AS FKC ON FK.object_id = FKC.constraint_object_id
                    INNER JOIN sys.columns AS C ON FKC.parent_object_id = C.object_id
                                                   AND FKC.parent_column_id = C.column_id
                    INNER JOIN sys.columns AS UC ON FKC.referenced_object_id = UC.object_id
                                                    AND FKC.referenced_column_id = UC.column_id
                    INNER JOIN sys.tables AS UT ON FKC.referenced_object_id = UT.object_id
                    INNER JOIN sys.schemas AS US ON UT.schema_id = US.schema_id
            WHERE   ( T.name <> 'sysdiagrams' )
            ORDER BY base_schema_name,
                    base_table_name
END


DECLARE @constraint_name NVARCHAR(128),
    @base_schema_name NVARCHAR(128),
    @base_table_name NVARCHAR(128),
    @unique_schema_name NVARCHAR(128),
    @unique_table_name NVARCHAR(128)

-- The foreign key side of the constraint is always singular, we need to check from the perspective
-- of the unique side of the constraint.

-- for each FK constraint in DB
DECLARE tmpC CURSOR READ_ONLY
    FOR SELECT DISTINCT
                constraint_name,
                base_schema_name,
                base_table_name,
                unique_schema_name,
                unique_table_name
        FROM    @foreign_key_columns

OPEN tmpC
FETCH NEXT FROM tmpC INTO @constraint_name, @base_schema_name, @base_table_name, @unique_schema_name, @unique_table_name
WHILE @@FETCH_STATUS = 0
    BEGIN
        -- get the columns in the base side of the FK constraint
        DECLARE @fkc TABLE
            (
              column_name NVARCHAR(128)
            )
        DELETE  FROM @fkc

        INSERT  INTO @fkc ( column_name )
                SELECT  base_column_name
                FROM    @foreign_key_columns
                WHERE   constraint_name = @constraint_name

        -- check for one to one/none
        -- If the base side columns of the constraint fit into any one of the base side tables unique constraints
        -- AND the column count is the same then we have a one-to-one/none and should be realized as a singular 
        -- object reference

        -- I realize that if the base side unique constraint has more columns than the unique side unique constraint
        -- AND all of those columns DO represent a 1:? that would actually qualify but it seems like an edge case and
        -- beyond the scope of this question.

        DECLARE @uk_schema_name NVARCHAR(128),
            @uk_table_name NVARCHAR(128),
            @uk_index_name NVARCHAR(128),
            @is_may_have_a BIT
        SET @is_may_have_a = 0

        -- have to open another cursor over the unique keys of the base table - i want
        -- a distinct list of unique constraints for the base table

        DECLARE cKey CURSOR READ_ONLY
            FOR SELECT  DISTINCT
                        [schema_name],
                        table_name,
                        index_name
                FROM    @unique_keys
                WHERE   [schema_name] = @base_schema_name
                        AND table_name = @base_table_name

        OPEN cKey
        FETCH NEXT FROM cKey INTO @uk_schema_name, @uk_table_name, @uk_index_name
        WHILE @@FETCH_STATUS = 0
            BEGIN

                -- get the unique constraint columns
                DECLARE @pkc TABLE
                    (
                      column_name NVARCHAR(128)
                    )
                DELETE  FROM @pkc

                INSERT  INTO @pkc ( column_name )
                        SELECT  column_name
                        FROM    @unique_keys
                        WHERE   [schema_name] = @uk_schema_name
                                AND table_name = @uk_table_name
                                AND index_name = @uk_index_name

                -- if count is same and columns are same
                DECLARE @count1 INT, @count2 INT
                SELECT  @count1 = COUNT(*) FROM    @fkc
                SELECT  @count2 = COUNT(*) FROM    @pkc

                IF @count1 = @count2 
                    BEGIN 
                        -- select all from both on name and exclude mismatches
                        SELECT  @count1 = COUNT(*)
                        FROM    @fkc F
                                FULL OUTER JOIN @pkc P ON f.column_name = p.column_name
                        WHERE   NOT p.column_name IS NULL AND NOT f.column_name IS NULL 

                        IF @count1 = @count2 
                            BEGIN
                                -- the base side of the fk constraint corresponds exactly to 
                                -- at least on unique constraint making it effectively 1:?
                                SET @is_may_have_a = 1
                                BREAK
                            END
                    END
                FETCH NEXT FROM cKey INTO @uk_schema_name, @uk_table_name, @uk_index_name
            END

        CLOSE cKey
        DEALLOCATE cKey

        IF @is_may_have_a = 1 
            PRINT 'for ' + @unique_schema_name + '.' + @unique_table_name + ' constraint ' + + @constraint_name + ' is 1:? ' 

        FETCH NEXT FROM tmpC INTO @constraint_name, @base_schema_name, @base_table_name, @unique_schema_name, @unique_table_name
    END

CLOSE tmpC
DEALLOCATE tmpC

For a look at the results on a more elaborate test db see TSQL: Identify a 1:? relationship