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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 ?
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If you don't need to omit any rows, you can use SQL Server's TOP clause to limit the rows returned. It is placed immediately after SELECT. The TOP keyword is followed by integer indicating the number of rows to return. In our example, we ordered by price and then limited the returned rows to 3.
While the table name is selected type CTRL + 3 and you will notice that the query will run and will return a single row as a resultset. Now developer just has to select the table name and click on CTRL + 3 or your preferred shortcut key and you will be able to see a single row from your table.
Use the view table_constraints in the information_schema schema. The column table_name gives you the name of the table in which the constraint is defined, and the column constraint_name contains the name of the constraint.
SQL constraints are used to specify rules for the data in a table. Constraints are used to limit the type of data that can go into a table. This ensures the accuracy and reliability of the data in the table. If there is any violation between the constraint and the data action, the action is aborted.
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.
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