As a data designer, data modeler, or data programmer, you may often come across the need to save a list of single values or parameters that are valid for the entire application. These values could be dates, booleans, numbers, or strings. In SQL Server, one common approach to store these values is by using a single row table.

A single row table is a table that consists of only one row. It is typically used to store variables or parameters that are frequently accessed or updated by the application. Examples of such variables include LastUpdate, CreationDate, statusIsActive, isInTest, and more.

When working with single row tables, it is important to remember that the table should always have one and only one row. If additional rows are added to the table, it can lead to unexpected results and behavior.

One way to ensure the singleness of a single row table is by using a calculated column as the primary key. By declaring a calculated column with a fixed value as the primary key, you can restrict the table to have only one row. Here’s an example:

CREATE TABLE dbo._params(
   paramsID as 1 PERSISTED CONSTRAINT PK_Params PRIMARY KEY,
   statusIsActive bit NOT NULL,
   isInTest bit NOT NULL,
   lastUpdate datetime NOT NULL,
   creationDate datetime NOT NULL,
   currentUSDEUR_ExchangeRate money NOT NULL,
   currentLanguage nchar(2) NOT NULL
)

In the above example, the paramsID column is a calculated column with a fixed value of 1. It is declared as PERSISTED and used as the primary key. This ensures that the table can only have one row.

By using a single row table with a calculated column as the primary key, you can prevent accidental or intentional insertion of new rows into the table. This can be particularly useful when the table is updated by power users through web forms or other means.

Remember, when working with single row tables, it is crucial to ensure the table remains with only one row. Adding additional rows can lead to unexpected results and behavior. By using a calculated column as the primary key, you can enforce the singleness of the table and maintain data integrity.