Slide 5.18: SQL SELECT statement

Slide 5.17: SQL (cont.)
Slide 5.19: SQL SELECT statement (cont.)
Home

SQL SELECT Statement

SELECT (Cont.)

Executing a SELECT command is basically done in three steps:
1. Derive a Cartesian product of the operand tables indicated in the FROM clause.
2. Based on this intermediate result, evaluate the conditions stated in the WHERE clause.
3. Project the result from Step 2 onto the attributes indicated in the SELECT clause; DISTINCT may be added to avoid duplicate tuples in the result.

SELECT Examples
Some of SELECT examples are given next:

Let R be a relational schema with a set of attributes X, where {A₁, ..., A_k} is contained in X. The project of R onto {A₁, ..., A_k} is expressed by
SELECT DISTINCT A1, ... Ak FROM R SQL> SELECT DISTINCT class FROM student;

Let R and S be relational schemata with equal sets of attributes. The union of R and S is expressed by

SELECT DISTINCT * FROM R  UNION  SELECT DISTINCT * FROM S

SQL> SELECT DISTINCT * FROM ( SELECT last_name FROM student )
  2    UNION
  3  SELECT DISTINCT * FROM ( SELECT first_name FROM student );

Slide 5.17: SQL (cont.)
Slide 5.19: SQL SELECT statement (cont.)
Home

“The secret of life, though, is to fall seven times and to get up eight times.”
― Paulo Coelho, The Alchemist