QL_statement ::= select_clause from_clause
[where_clause][groupby_clause][having_clause][orderby_clause]
Java Platform, Enterprise Edition (Java EE) 8 The Java EE Tutorial |
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This section briefly describes the syntax of the query language so that you can quickly move on to Example Queries. When you are ready to learn about the syntax in more detail, see Full Query Language Syntax.
The following topics are addressed here:
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 BNF syntax defines the following clauses.
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.
Update and delete statements provide bulk operations over sets of entities. These statements have the following syntax:
update_statement :: = update_clause [where_clause]
delete_statement :: = delete_clause [where_clause]
The update and delete clauses determine the type of the entities to be
updated or deleted. The WHERE
clause may be used to restrict the scope
of the update or delete operation.
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