Select Statements

A select query has six clauses: SELECT, FROM, WHERE, GROUP BY, HAVING, and ORDER BY. The SELECT and FROM clauses are required, but the WHERE, GROUP BY, HAVING, and ORDER BY clauses are optional. Here is the high-level BNF syntax of a query language select query:

QL_statement ::= select_clause from_clause 
  [where_clause][groupby_clause][having_clause][orderby_clause]

• The SELECT clause defines the types of the objects or values returned by the query.

• The FROM clause defines the scope of the query by declaring one or more identification variables, which can be referenced in the SELECT and WHERE clauses. An identification variable represents one of the following elements:

  • The abstract schema name of an entity

  • An element of a collection relationship

  • An element of a single-valued relationship

  • A member of a collection that is the multiple side of a one-to-many relationship

• The WHERE clause is a conditional expression that restricts the objects or values retrieved by the query. Although the clause is optional, most queries have a WHERE clause.

• The GROUP BY clause groups query results according to a set of properties.

• The HAVING clause is used with the GROUP BY clause to further restrict the query results according to a conditional expression.

• The ORDER BY clause sorts the objects or values returned by the query into a specified order.