ITPEC IP April 2024 Question 28
Topic: Databases
Which of the following is an appropriate combination of relational database manipulations I through III and relational operations?
IExtracting the specified field (column)IIExtracting the specified record (row)IIICombining multiple tables into one (1) table
Explanation
I Projection, II Selection, III Join — the three basic relational operations take columns, take rows, and combine tables.
- •Projection extracts the specified columns (fields) from every row, such as only ProductName and Price from a Product table: it cuts the table vertically
- •Selection extracts the rows (records) that satisfy a condition, such as the products with a price of 100 or more: it cuts the table horizontally
- •Join combines two or more tables into one through the columns they share, such as a foreign key in one table and the primary key of the other
Answer (c)
Why not others:
- •(a) — Join, Projection, Selection: every operation is on the wrong item; join extracts no column, and selection combines no tables
- •(b) — Projection, Join, Selection: I is right, but II and III are swapped; join does not extract rows
- •(d) — Selection, Projection, Join: III is right, but I and II are swapped; selection takes rows, not columns
Key rule: Projection chooses columns, selection chooses rows, join links tables; in SQL they are the column list after SELECT, the WHERE condition, and a JOIN of tables.
Databases · AI-written, may contain errors
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