Question

I have a table that looks something like this:

CREATE TABLE A (
  A_UNIQUE_ID int NOT NULL PRIMARY KEY,
  A_OTHER_ID int NOT NULL,
  A_CURRENT_FL bit NOT NULL )

and it has values similar to this

INSERT INTO TABLE A VALUES (1, 1, 0);
INSERT INTO TABLE A VALUES (2, 1, 0);
INSERT INTO TABLE A VALUES (3, 1, 1);

I want to create another table B like this

CREATE TABLE B (
  B_UNIQUE_ID int NOT NULL PRIMARY KEY,
  A_OTHER_ID int NOT NULL)

where B.A_OTHER_ID is constrained to a unique row in A. So B.A_OTHER_ID = A.A_OTHER_ID AND A.CURRENT_FL = 1. Is that possible with some type of check constraint setup? Examples are very much appreciated.

Answer 1

You can do it with some computed columns, but it doesn’t look very pretty:

CREATE TABLE A (
  A_UNIQUE_ID int NOT NULL PRIMARY KEY,
  A_OTHER_ID int NOT NULL,
  A_CURRENT_FL bit NOT NULL,
  OTHER_XREF as CASE WHEN A_CURRENT_FL = 1 THEN A_OTHER_ID END persisted,
  UNIQ as CASE WHEN A_CURRENT_FL = 0 THEN A_UNIQUE_ID ELSE 0 END persisted 
  constraint UQ_OTHER_XREF UNIQUE (OTHER_XREF,UNIQ)
  )

INSERT INTO A VALUES (1, 1, 0);
INSERT INTO A VALUES (2, 1, 0);
INSERT INTO A VALUES (3, 1, 1);

CREATE TABLE B (
  B_UNIQUE_ID int NOT NULL PRIMARY KEY,
  A_OTHER_ID int NOT NULL,
  XREF as 0 persisted,
  constraint FK_B_A FOREIGN KEY (A_OTHER_ID,XREF) references A (OTHER_XREF,UNIQ))

This works:

  insert into B VALUES (2,1)

This breaks the FK constraint:

  insert into B VALUES (3,2)

The two computed columns in table A ensure that any row with A_CURRENT_FL = 1 has that as a distinct value in OTHER_XREF. Other rows will have NULL in that column. However, to apply a UNIQUE constraint (the target for the foreign key), we need something that will be distinctly different for every row with a NULL, whilst being a well-known value for the row with A_CURRENT_FL = 1. In this case, I made the well known value 0.

Finally, the foreign key constraint needs to match both of these columns, so we add a new computed column in table B with the well known value 0.

Column names aren’t particularly good, but I’m sure you can come up with some.

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Note, also, that by the above, we’ve also implemented another constraint that you didn’t mention, but that also may be important – there can’t be two rows now in table A that both have A_CURRENT_FL set to 1, for the same A_OTHER_ID value. This also generates an error:

INSERT INTO A VALUES (4,1,1);