What is the selection operation in DBMS?

A selection operator $(\sigma)$ is a unary operator in relational algebra. We use it to select the relation's records or rows that satisfy its condition(s).

Syntax

Parameters

In the syntax above:

• $\sigma$ denotes the selection operation.

• $condtion$ denotes any relational conditions. Relational conditions can be anything like$[= , !=, > ,<, <=,>=]$.

• $relation$ denotes the table from the database onto which the selection operation is being performed.

Example

Consider a case where we have a relation $Persons$(Id, Name, Hometown, Age) as shown below:

The query below will return a table having only the records where the Age is equal to $22$.

The query's output from above will be as follows:

Multiple relational conditions

We can also have multiple relational conditions in the selection operation to get desired records. Multiple conditions join with logical operations (AND, OR). For instance:

The query above will select only those rows or records with age equal to $22$ or hometown equal to Texas. The output will be as follows:

Conclusion

The selection operation in DBMS (Database Management System) allows us to retrieve specific records or rows from a relation based on certain conditions. Using logical operators such as AND and OR, we can combine multiple relational conditions to refine our selection and obtain the desired subset of data.