FROM

Artikel
08/29/2011

Specifies the collection used in SELECT statements.

FROM expression [ ,...n ] as C

Arguments

expression
Any valid query expression that yields a collection to use as a source in a SELECT statement.

Remarks

A FROM clause is a comma-separated list of one or more FROM clause items. The FROM clause can be used to specify one or more sources for a SELECT statement. The simplest form of a FROM clause is a single query expression that identifies a collection and an alias used as the source in a SELECT statement, as illustrated in the following example:

FROM C as c

FROM Clause Items

Each FROM clause item refers to a source collection in the Entity SQL query. Entity SQL supports the following classes of FROM clause items: simple FROM clause items, JOIN FROM clause items, and APPLY FROM clause items. Each of these FROM clause items is described in more detail in the following sections.

Simple FROM Clause Item

The simplest FROM clause item is a single expression that identifies a collection and an alias. The expression can simply be an entity set, or a subquery, or any other expression that is a collection type. The following is an example:

LOB.Customers as c

The alias specification is optional. An alternate specification of the above from clause item could be the following:

LOB.Customers

If no alias is specified, Entity SQL attempts to generate an alias based on the collection expression.

JOIN FROM Clause Item

A JOIN FROM clause item represents a join between two FROM clause items. Entity SQL supports cross joins, inner joins, left and right outer joins, and full outer joins. All these joins are supported similar to the way that they are supported in Transact-SQL. As in Transact-SQL, the two FROM clause items involved in the JOIN must be independent. That is, they cannot be correlated. A CROSS APPLY or OUTER APPLY can be used for these cases.

Cross Joins

A CROSS JOIN query expression produces the Cartesian product of the two collections, as illustrated in the following example:

FROM C AS c CROSS JOIN D as d

Inner Joins

An INNER JOIN produces a constrained Cartesian product of the two collections, as illustrated in the following example:

FROM C AS c [INNER] JOIN D AS d ON e

Left Outer Joins and Right Outer Joins

An OUTER JOIN query expression produces a constrained Cartesian product of the two collections, as illustrated in the following example:

FROM C AS c LEFT OUTER JOIN D AS d ON e

The previous query expression processes a combination of every element of the collection on the left paired against every element of the collection on the right, where the ON condition is true. If the ON condition is false, the expression still processes a single instance of the element on the left paired against the element on the right, with the value null.

A RIGHT OUTER JOIN may be expressed in a similar manner.

Full Outer Joins

An explicit FULL OUTER JOIN produces a constrained Cartesian product of the two collections as illustrated in the following example:

FROM C AS c FULL OUTER JOIN D AS d ON e

The previous query expression processes a combination of every element of the collection on the left paired against every element of the collection on the right, where the ON condition is true. If the ON condition is false, the expression still processes one instance of the element on the left paired against the element on the right, with the value null. It also processes one instance of the element on the right paired against the element on the left, with the value null.

Note

To preserve compatibility with SQL-92, in Transact-SQL the OUTER keyword is optional. Therefore, LEFT JOIN, RIGHT JOIN, and FULL JOIN are synonyms for LEFT OUTER JOIN, RIGHT OUTER JOIN, and FULL OUTER JOIN.

APPLY Clause Item

Entity SQL supports two kinds of APPLY: CROSS APPLY and OUTER APPLY.

A CROSS APPLY produces a unique pairing of each element of the collection on the left with an element of the collection produced by evaluating the expression on the right. With a CROSS APPLY, the expression on the right is functionally dependent on the element on the left, as illustrated in the following associated collection example:

SELECT c, f FROM C AS c CROSS APPLY c.Assoc AS f

The behavior of CROSS APPLY is similar to the join list. If the expression on the right evaluates to an empty collection, the CROSS APPLY produces no pairings for that instance of the element on the left.

An OUTER APPLY resembles a CROSS APPLY, except a pairing is still produced even when the expression on the right evaluates to an empty collection. The following is an example of an OUTER APPLY:

SELECT c, f FROM C AS c OUTER APPLY c.Assoc AS f

Note

Unlike in Transact-SQL, there is no need for an explicit unnest step in Entity SQL.

Note:
CROSS and OUTER APPLY operators were introduced in SQL Server 2005. In some cases, the query pipeline might produce Transact-SQL that contains CROSS APPLY and/or OUTER APPLY operators. Because some backend providers, including versions of SQL Server earlier than SQL Server 2005, do not support these operators, such queries cannot be executed on these backend providers. Some typical scenarios that might lead to the presence of CROSS APPLY and/or OUTER APPLY operators in the output query are the following: a correlated subquery with paging; AnyElement over a correlated subquery or over a collection produced by navigation; LINQ queries that use grouping methods that accept an element selector; a query in which a CROSS APPLY or an OUTER APPLY are explicitly specified; a query that has a DEREF construct over a REF construct.

CROSS and OUTER APPLY operators were introduced in SQL Server 2005. In some cases, the query pipeline might produce Transact-SQL that contains CROSS APPLY and/or OUTER APPLY operators. Because some backend providers, including versions of SQL Server earlier than SQL Server 2005, do not support these operators, such queries cannot be executed on these backend providers.

Some typical scenarios that might lead to the presence of CROSS APPLY and/or OUTER APPLY operators in the output query are the following: a correlated subquery with paging; AnyElement over a correlated subquery or over a collection produced by navigation; LINQ queries that use grouping methods that accept an element selector; a query in which a CROSS APPLY or an OUTER APPLY are explicitly specified; a query that has a DEREF construct over a REF construct.

Multiple Collections in the FROM Clause

The FROM clause can contain more than one collection separated by commas. In these cases, the collections are assumed to be joined together. Think of these as an n-way CROSS JOIN.

In the following example,CandDare independent collections, butc.Namesis dependent on C.

FROM C AS c, D AS d, c.Names AS e

The previous example is logically equivalent to the following example:

FROM (C AS c JOIN D AS d) CROSS APPLY c.Names AS e

Left Correlation

Items in the FROM clause can refer to items specified in earlier clauses. In the following example, C and D are independent collections, but c.Names is dependent on C:

from C as c, D as d, c.Names as e

This is logically equivalent to:

from (C as c join D as d) cross apply c.Names as e

Semantics

Logically, the collections in the FROM clause are assumed to be part of an n-way cross join (except in the case of a 1-way cross join). Aliases in the FROM clause are processed left to right, and are added to the current scope for later reference. The FROM clause is assumed to produce a multiset of rows. There will be one field for each item in the FROM clause that represents a single element from that collection item.

The FROM clause logically produces a multiset of rows of type Row(c, d, e) where fields c, d, and e are assumed to be of the element type of C, D, and c.Names.

Entity SQL introduces an alias for each simple FROM clause item in scope. For example, in the following FROM clause snippet, The names introduced into scope are c, d, and e.

from (C as c join D as d) cross apply c.Names as e

In Entity SQL (unlike Transact-SQL), the FROM clause only introduces the aliases into scope. Any references to columns (properties) of these collections must be qualified with the alias.

Pulling Up Keys from Nested Queries

Certain types of queries that require pulling up keys from a nested query are not supported. For example, the following query is valid:

select c.Orders from Customers as c

However, the following query is not valid, because the nested query does not have any keys:

select {1} from {2, 3}

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