Structured query language (SQL): SELECT

The idea

SELECT is the clause that tells a database which columns you want returned. In AQA GCSE Computer Science you read and write simple SQL against a named table. After SELECT you list the field names, separated by commas, and the query returns those fields for every matching record. Writing SELECT * returns every field in the table, which is useful when you need the whole record but wasteful when you need one column. For example, SELECT Name, Score FROM Results returns just those two fields. SELECT does not filter rows (that is WHERE) and does not choose the table (that is FROM); it only chooses the output fields, so keep its job separate when answering multiple-choice questions.

What earns the marks

• Identifies SELECT as the clause that specifies which fields (columns) are returned by the query.
• Recognises that SELECT * returns all fields in the table.
• Distinguishes SELECT (choosing output fields) from FROM (choosing the table) and WHERE (filtering rows).
• Accepts that field names follow SELECT and are separated by commas when more than one is required.

Common mistakes and how to put them right

• Thinking SELECT chooses the table.

  Put it right: The table is named after FROM, while SELECT lists the fields to return.

• Believing SELECT filters records.

  Put it right: Row filtering is done by WHERE; SELECT only decides which columns appear in the output.

• Assuming SELECT * returns only one column.

  Put it right: The asterisk means all fields are returned.

Exam tips

• Read the whole query before choosing an answer, and check which word names the fields and which names the table.
• Remember the order SELECT ... FROM ... WHERE and match each keyword to its single job.
• If a question asks what is returned, trace the field list after SELECT rather than guessing from the table name.