Single OPERAND operations are known as unary operations in Relational Algebra. In relational algebra, the operations PROJECTION and SELECTION and RENAME are unary operations:

• SELECT: The SELECT operation is used to select a subset of TUPLES from a relation that meets a set of criteria. The SELECT operation can be thought of as a filter that keeps only those tuples that meet a set of criteria. Alternatively, the SELECT operation can be used to limit the tuples in relation to those that meet the criterion. The SELECT action can also be thought of as a horizontal division of the relation into two sets of tuples: those that fulfil the condition and are selected, and those that do not and are rejected.
• PROJECT: If we consider a relation to be a table, the SELECT operation selects some of the rows while discarding OTHERS. The PROJECT operation, on the other hand, picks a subset of the table's COLUMNS while discarding the rest. If we're just interested in a few of a relation's attributes, we may use the PROJECT operation to project the relation over just those. As a result, the PROJECT operation's result can be seen as a vertical division of the relation into two relations: one with the required columns (attributes) and the operation's result, and the other with the rejected columns.
• RENAME: Breaking a difficult chain of operations and renaming it as a relation with new names is sometimes straightforward and appropriate. There are numerous reasons to rename a relation, including:
  • It is possible that we will wish to save the outcome of a relational algebra expression as a relation so we may use it later.
  • We may want to join relation to itself.