How Entity SQL Differs from Transact-SQL

Artikel
04/15/2009

This topic describes the differences between Entity SQL and Transact-SQL.

Inheritance and Relationships Support

Entity SQL works directly with conceptual entity schemas and supports EDM features such as inheritance and relationships.

When working with inheritance, it is often useful to select instances of a subtype from a collection of supertype instances. The oftype operator in Entity SQL (similar to oftype in C# Sequences) provides this capability. For information about this and other type operators, see Type Operators.

Support for Collections

Entity SQL treats collections as first-class entities. For example:

Collection expressions are valid in a from clause.
in and exists subqueries have been generalized to allow any collections.

A subquery is one kind of collection. e1 in e2 and exists(e) are the Entity SQL constructs to perform these operations.
Set operations, such as union, intersect, and except, now operate on collections.
Joins operate on collections.

Suport for Expressions

Transact-SQL has subqueries (tables) and expressions (rows and columns).

To support collections and nested collections, Entity SQL makes everything an expression. Entity SQL is more composable than Transact-SQL—every expression can be used anywhere. Query expressions always result in collections of the projected types and can be used anywhere a collection expression is allowed. For information about Transact-SQL expressions that are not supported in Entity SQL, see Unsupported Expressions (Entity SQL).

The following are all valid Entity SQL queries:

1+2 *3
"abc"
row(1 as a, 2 as b)
{ 1, 3, 5} 
e1 union all e2
set(e1)

Uniform Treatment of Subqueries

Given its emphasis on tables, Transact-SQL performs contextual interpretation of subqueries. For example, a subquery in the from clause is considered to be a multiset (table). But the same subquery used in the select clause is considered to be a scalar subquery. Similarly, a subquery used on the left side of an in operator is considered to be a scalar subquery, while the right side is expected to be a multiset subquery.

Entity SQL eliminates these differences. An expression has a uniform interpretation that does not depend on the context in which it is used. Entity SQL considers all subqueries to be multiset subqueries. If a scalar value is desired from the subquery, Entity SQL provides the anyelement operator that operates on a collection (in this case, the subquery), and extracts a singleton value from the collection.

Avoiding Implicit Coercions for Subqueries

A related side effect of uniform treatment of subqueries is implicit conversion of subqueries to scalar values. Specifically, in Transact-SQL, a multiset of rows (with a single field) is implicitly converted into a scalar value whose datatype is that of the field.

Entity SQL does not support this implicit coercion. Entity SQL provides the ANYELEMENT operator to extract a singleton value from a collection, and a select value clause to avoid creating a row-wrapper during a query expression.

Select Value: Avoiding the Implicit Row Wrapper

Transact-SQL is ambiguous about the treatment of the result from a query. The select clause in a Transact-SQL subquery implicitly creates a row wrapper around the items in the clause. This implies that we cannot create collections of scalars or objects. Transact-SQL allows an implicit coercion between a rowtype with one field, and a singleton value of the same datatype.

Entity SQL provides the select value clause to skip the implicit row construction. Only one item may be specified in a select value clause. When such a clause is used, no row wrapper is constructed around the items in the select clause, and a collection of the desired shape may be produced, for example: select value a.

Entity SQL also provides the row constructor to construct arbitrary rows. select takes one or more elements in the projection and results in a data record with fields, as follows:

select a, b, c

Left Correlation and Aliasing

In Transact-SQL, expressions in a given scope (a single clause like select or from) cannot reference expressions defined earlier in the same scope. Some dialects of SQL (including Transact-SQL) do support limited forms of these in the from clause.

Entity SQL generalizes left correlations in the from clause, and treats them uniformly. Expressions in the from clause can reference earlier definitions (definitions to the left) in the same clause without the need for additional syntax.

Entity SQL also imposes additional restrictions on queries involving group by clauses. Expressions in the select clause and having clause of such queries may only refer to the group by keys via their aliases. The following construct is valid in Transact-SQL but are not in Entity SQL:

select t.x + t.y from T as t group by t.x + t.y

To do this in Entity SQL:

select k from T as t group by (t.x + t.y) as k

Referencing Columns (Properties) of Tables (Collections)

All column references in Entity SQL must be qualified with the table alias. The following construct (assuming that a is a valid column of table T) are valid in Transact-SQL but not in Entity SQL.

select a from T

The Entity SQL form is

select t.a as A from T as t

The table aliases are optional in the from clause. The name of the table is used as the implicit alias. Entity SQL allows the following form as well:

select Tab.a from Tab

Transact-SQL uses the "." notation for referencing columns of (a row of) a table. Entity SQL extends this notation (borrowed from programming languages) to support navigation through properties of an object.

For example, if p is an expression of type Person, the following is the Entity SQL syntax for referencing the city of the address of this person.

p.Address.City

No Support for *

Transact-SQL supports the unqualified * syntax as an alias for the entire row, and the qualified * syntax (t.*) as a shortcut for the fields of that table. In addition, Transact-SQL allows for a special count(*) aggregate, which includes nulls.

Entity SQL does not support the * construct. Transact-SQL queries of the form select * from T and select T1.* from T1, T2... can be expressed in Entity SQL as select value t from T as t and select value t1 from T1 as t1, T2 as t2..., respectively. Additionally, these constructs handle inheritance (value substitutability), while the select * variants are restricted to top-level properties of the declared type.

Entity SQL does not support the count(*) aggregate. Use count(0) instead.

Changes to Group By

Entity SQL supports aliasing of group by keys. Expressions in the select clause and having clause must refer to the group by keys via these aliases. For example, this Entity SQL syntax:

select k1, count(t.a), sum(t.a)
from T as t
group by t.b + t.c as k1

...is equivalent to the following Transact-SQL:

select b + c, count(*), sum(a) 
from T
group by b + c

Collection-Based Aggregates

Entity SQL supports two kinds of aggregates.

Collection-based aggregates operate on collections and produce the aggregated result. These can appear anywhere in the query, and do not require a group by clause. For example:

select t.a as a, count({1,2,3}) as b from T as t

Entity SQL also supports SQL-style aggregates. For example:

select a, sum(t.b) from T as t group by t.a as a

ORDER BY Clause Usage

Transact-SQL allows ORDER BY clauses to be specified only in the topmost SELECT .. FROM .. WHERE block. In Entity SQL you can use a nested ORDER BY expression and it can be placed anywhere in the query, but ordering in a nested query is not preserved.

-- The following query will order the results by the last name
SELECT C1.FirstName, C1.LastName
        FROM AdventureWorks.Contact as C1
        ORDER BY C1.LastName

-- In the following query ordering of the nested query is ignored.
SELECT C2.FirstName, C2.LastName
    FROM (SELECT C1.FirstName, C1.LastName
        FROM AdventureWorks.Contact as C1
        ORDER BY C1.LastName) as C2

Identifiers

In Transact-SQL, identifier comparison is based on the collation of the current database. In Entity SQL, identifiers are always case insensitive and accent sensitive (that is, Entity SQL distinguishes between accented and unaccented characters; for example, 'a' is not equal to 'ấ'). Entity SQL treats versions of letters that appear the same but are from different code pages as different characters. For more information, see Input Character Set (Entity SQL).

Transact-SQL Functionality Not Available in Entity SQL

The following Transact-SQL functionality is not available in Entity SQL.

DML
Entity SQL currently provides no support for DML statements (insert, update, delete).

DDL
Entity SQL provides no support for DDL in the current version.

Imperative Programming
Entity SQL provides no support for imperative programming, unlike Transact-SQL. Use a programming language instead.

Grouping Functions
Entity SQL does not yet provide support for grouping functions (for example, CUBE, ROLLUP, and GROUPING_SET).

Analytic Functions
Entity SQL does not (yet) provide support for analytic functions.

Built in Functions, Operators
Entity SQL supports a subset of Transact-SQL's built in functions and operators. These operators and functions are likely to be supported by the major store providers. Entity SQL uses the store-specific functions declared in a provider manifest. Additionally, the entity data model allows you to declare built-in and user-defined existing store functions, for Entity SQL to use.

Hints
Entity SQL does not provide mechanisms for query hints.

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