How to constrain a table to contain a single row?

Question

I want to store a single row in a configuration table for my application. I would like to enforce that this table can contain only one row.

What is the simplest way to enforce the single row constraint ?

Stendhal

Why not use a table with columns (Name, Value) with a primary key on Name. Then you can select Value from Table where Name = ? with certainty that either no rows or one row will be returned. – Rete

I'm not sure sql is the best solution here. Maybe a simple xml file is more appropriate for configuration. I use to think that configuration != data and sql was made for data. – Timework

@ar - I've seen that go badly wrong when you're expecting to read, say, an integer, and you get some badly formatted value in the value column. – Rolf

@Rolf Why would that happen? Because you specified a nonexistent value for Name? – Ternan

@Ternan - note that my comment was a response to ars comment. The issue is, if you're just storing name/value pairs, the value pretty well has to be string, and you've got no means of enforcing validation in the database. When you use a single-row table with separate columns for each setting (as the OP wanted) then you can easily enforce validation for each configuration setting via check constraints. – Rolf

Answer 1

You make sure one of the columns can only contain one value, and then make that the primary key (or apply a uniqueness constraint).

CREATE TABLE T1(
    Lock char(1) not null,
    /* Other columns */,
    constraint PK_T1 PRIMARY KEY (Lock),
    constraint CK_T1_Locked CHECK (Lock='X')
)

I have a number of these tables in various databases, mostly for storing config. It's a lot nicer knowing that, if the config item should be an int, you'll only ever read an int from the DB.

Rolf

Here's a follow up question to think over. What is the primary key of this table? :) – Softball

@dportas - I believe it's natural key would be {}, but we're not allowed to implement that in SQL. – Rolf

Exactly so! By definition Lock cannot be a key because a key must be irreducible. But SQL has no syntax that allows for the empty set being a key. So in this case PRIMARY KEY != primary key. – Softball

@dportas - I'd be a fraud if I didn't admit to reading c.d.t and TTM, among other sources, so your question was a bit of a gimme. – Rolf

@Rolf what is c.d.t and TTM? Thanks for sharing your resources so we can learn :) – Anagnos

@BZ - c.d.t is shorthand for comp.databases.theory, a usenet group (visible through Google groups) that I admit I haven't read much of recently. It was more oriented around relational theory than SQL - but I happened to know that dportas/sqlvogel also frequented the same group. TTM was a reference to The Third Manifesto, which is a good book talking (again) about relational theory rather than SQL. – Rolf

Has the SQL standards group considered making it legal to alter table t add primary key ()? – Dariodariole

Answer 2

I usually use Damien's approach, which has always worked great for me, but I also add one thing:

CREATE TABLE T1(
    Lock char(1) not null DEFAULT 'X',
    /* Other columns */,
    constraint PK_T1 PRIMARY KEY (Lock),
    constraint CK_T1_Locked CHECK (Lock='X')
)

Adding the "DEFAULT 'X'", you will never have to deal with the Lock column, and won't have to remember which was the lock value when loading the table for the first time.

Piperpiperaceous

The default constraint should also be named or else it will get a autogenerated confusing name. Lock char(1) not null CONSTRAINT DF_T1_Lock DEFAULT 'X' – Mornings

Further, you should make your default value something other than 'X'. I made mine a longer string, and this is my error message now if I attempt to insert a second row: Violation of PRIMARY KEY constraint 'PK_RestrictToOneRow'. Cannot insert duplicate key in object 'dbo.1ROWTABLE'. The duplicate key value is (This table is locked to one row). – Gordon

Answer 3

You may want to rethink this strategy. In similar situations, I've often found it invaluable to leave the old configuration rows lying around for historical information.

To do that, you actually have an extra column creation_date_time (date/time of insertion or update) and an insert or insert/update trigger which will populate it correctly with the current date/time.

Then, in order to get your current configuration, you use something like:

select * from config_table order by creation_date_time desc fetch first row only

(depending on your DBMS flavour).

That way, you still get to maintain the history for recovery purposes (you can institute cleanup procedures if the table gets too big but this is unlikely) and you still get to work with the latest configuration.

Nutty

Answer 4

You can implement an INSTEAD OF Trigger to enforce this type of business logic within the database.

The trigger can contain logic to check if a record already exists in the table and if so, ROLLBACK the Insert.

Now, taking a step back to look at the bigger picture, I wonder if perhaps there is an alternative and more suitable way for you to store this information, perhaps in a configuration file or environment variable for example?

Maggot

Answer 5

I know this is very old but instead of thinking BIG sometimes better think small use an identity integer like this:

Create Table TableWhatever
(
    keycol int primary key not null identity(1,1) 
         check(keycol =1),
    Col2 varchar(7)
)

This way each time you try to insert another row the check constraint will raise preventing you from inserting any row since the identity p key won't accept any value but 1

Marvel

Old question/answer, I know. Problem with this solution that you can only insert into this row once. If the record is accidentally deleted, you can't re-insert anything into this, as next value for Identity is 2 and that does not pass check. – Theosophy

Answer 6

Here's a solution I came up with for a lock-type table which can contain only one row, holding a Y or N (an application lock state, for example).

Create the table with one column. I put a check constraint on the one column so that only a Y or N can be put in it. (Or 1 or 0, or whatever)

Insert one row in the table, with the "normal" state (e.g. N means not locked)

Then create an INSERT trigger on the table that only has a SIGNAL (DB2) or RAISERROR (SQL Server) or RAISE_APPLICATION_ERROR (Oracle). This makes it so application code can update the table, but any INSERT fails.

DB2 example:

create table PRICE_LIST_LOCK
(
    LOCKED_YN       char(1)   not null  
        constraint PRICE_LIST_LOCK_YN_CK  check (LOCKED_YN in ('Y', 'N') )
);
--- do this insert when creating the table
insert into PRICE_LIST_LOCK
values ('N');

--- once there is one row in the table, create this trigger
CREATE TRIGGER ONLY_ONE_ROW_IN_PRICE_LIST_LOCK
   NO CASCADE 
   BEFORE INSERT ON PRICE_LIST_LOCK
   FOR EACH ROW
   SIGNAL SQLSTATE '81000'  -- arbitrary user-defined value
     SET MESSAGE_TEXT='Only one row is allowed in this table';

Works for me.

Indelicate

Answer 7

I use a bit field for primary key with name IsActive. So there can be 2 rows at most and and the sql to get the valid row is: select * from Settings where IsActive = 1 if the table is named Settings.

Co

Answer 8

The easiest way is to define the ID field as a computed column by value 1 (or any number ,....), then consider a unique index for the ID.

CREATE TABLE [dbo].[SingleRowTable](
  [ID]  AS ((1)),
  [Title] [varchar](50) NOT NULL,
   CONSTRAINT [IX_SingleRowTable] UNIQUE NONCLUSTERED 
   (
      [ID] ASC
   )
) ON [PRIMARY]

Floorman

Answer 9

You can write a trigger on the insert action on the table. Whenever someone tries to insert a new row in the table, fire away the logic of removing the latest row in the insert trigger code.

Haff

Answer 10

Old question but how about using IDENTITY(MAX,1) of a small column type?

CREATE TABLE [dbo].[Config](
[ID] [tinyint] IDENTITY(255,1) NOT NULL,
[Config1] [nvarchar](max) NOT NULL,
[Config2] [nvarchar](max) NOT NULL

Lipocaic

You could add another row by using SET IDENTITY_INSERT. – Adeline

Answer 11

Create an id of type ENUM('') NOT NULL PRIMARY KEY.
The ENUM('') gives you only 1 option.
The NOT NULL means that the null cannot happen.

  CREATE TABLE T1(
        id ENUM('') NOT NULL PRIMARY KEY,
      /* Other columns */,
  )

Logjam

This answer was reviewed in the Low Quality Queue. Here are some guidelines for How do I write a good answer?. Code only answers are not considered good answers, and are likely to be downvoted and/or deleted because they are less useful to a community of learners. It's only obvious to you. Explain what it does, and how it's different / better than existing answers. From Review – Coray

Answer 12

IF NOT EXISTS ( select * from table )
BEGIN
    ///Your insert statement
END

Unbelieving

Answer 13

Here we can also make an invisible value which will be the same after first entry in the database.Example: Student Table: Id:int firstname:char Here in the entry box,we have to specify the same value for id column which will restrict as after first entry other than writing lock bla bla due to primary key constraint thus having only one row forever. Hope this helps!

Welton