Select (SQL)

From Academic Kids

A SELECT statement in SQL returns a result set of records from one or more tables.

It is used to retrieve zero or more rows from one or more tables in a database. In most applications, SELECT is the most commonly used Data Manipulation Language (DML) command. In specifying a SELECT query, the user specifies a description of the desired result set, but they do not specify what physical operations must be executed to produce that result set. Translating the query into an optimal "query plan" is left to the database system, more specifically to the query optimiser.

Commonly available keywords related to SELECT include:

  • WHERE – used to identify which rows to be retrieved, or applied to GROUP BY.
  • GROUP BY – used to combine rows with related values into elements of a smaller set of rows.
  • HAVING – used to identify which rows, following a GROUP BY, are to be retrieved.
  • ORDER BY – used to identify which columns are used to sort the resulting data.

Examples

Table "T" Query Result
C1 C2
1 a
2 b
SELECT * FROM T;
C1 C2
1 a
2 b
C1 C2
1 a
2 b
SELECT C1 FROM T;
C1
1
2
C1 C2
1 a
2 b
SELECT * FROM T WHERE C1 = 1;
C1 C2
1 a

Given a table T, the query SELECT * FROM T; will result in all the elements of all the rows of the table being shown.

With the same table, the query SELECT C1 FROM T; will result in the elements from the column C1 of all the rows of the table being shown — in Relational algebra terms, a projection will be performed.

With the same table, the query SELECT * FROM T WHERE C1 = 1; will result in all the elements of all the rows where the value of column C1 is '1' being shown — in Relational algebra terms, a selection will be performed, because of the WHERE keyword.


See also

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