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Computer Science

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Figure 8.11: Selection operation in a relational database

The projection is defined as the set obtained when the components of the tuples are
restricted to the set, it discards (or excludes) the other attributes. It extracts only the
specified relation attributes from a tuple or set of tuples. It is a vertical subset of
table columns, and it may include one column, two columns or all table columns.
Projection answers the question: which columns shall the query return? For example,
`Select empno, fname, job From emp`; returns columns as shown in Figure 8.12.

Figure 8.12: Projection operation in the relational database

Projection differs from selection as SELECT is used to select all rows of a specific
relation that meet a particular condition, whereas the project is used to select
particular columns. However, one query may contain both: selection and projection:
For example, `select empno, fname, job From emp Where deptno=10`; returns
enclosed columns and rows as in Figure 8.13.

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Computer Science Form 5.indd 448 23/07/2024 12:34