Общие табличные выражения sql
Обновлено: 21.11.2024
Specifies a temporary named result set, known as a common table expression (CTE). This is derived from a simple query and defined within the execution scope of a single SELECT, INSERT, UPDATE, DELETE or MERGE statement. This clause can also be used in a CREATE VIEW statement as part of its defining SELECT statement. A common table expression can include references to itself. This is referred to as a recursive common table expression.
Syntax
To view Transact-SQL syntax for SQL Server 2014 and earlier, see Previous versions documentation.
Arguments
expression_name
Is a valid identifier for the common table expression. expression_name must be different from the name of any other common table expression defined in the same WITH <common_table_expression> clause, but expression_name can be the same as the name of a base table or view. Any reference to expression_name in the query uses the common table expression and not the base object.
column_name
Specifies a column name in the common table expression. Duplicate names within a single CTE definition are not allowed. The number of column names specified must match the number of columns in the result set of the CTE_query_definition. The list of column names is optional only if distinct names for all resulting columns are supplied in the query definition.
CTE_query_definition
Specifies a SELECT statement whose result set populates the common table expression. The SELECT statement for CTE_query_definition must meet the same requirements as for creating a view, except a CTE cannot define another CTE. For more information, see the Remarks section and CREATE VIEW (Transact-SQL).
If more than one CTE_query_definition is defined, the query definitions must be joined by one of these set operators: UNION ALL, UNION, EXCEPT, or INTERSECT.
Remarks
Guidelines for Creating and Using Common Table Expressions
The following guidelines apply to nonrecursive common table expressions. For guidelines that apply to recursive common table expressions, see Guidelines for Defining and Using Recursive Common Table Expressions that follows.
A CTE must be followed by a single SELECT , INSERT , UPDATE , or DELETE statement that references some or all the CTE columns. A CTE can also be specified in a CREATE VIEW statement as part of the defining SELECT statement of the view.
Multiple CTE query definitions can be defined in a nonrecursive CTE. The definitions must be combined by one of these set operators: UNION ALL , UNION , INTERSECT , or EXCEPT .
A CTE can reference itself and previously defined CTEs in the same WITH clause. Forward referencing is not allowed.
Specifying more than one WITH clause in a CTE is not allowed. For example, if a CTE_query_definition contains a subquery, that subquery cannot contain a nested WITH clause that defines another CTE.
The following clauses cannot be used in the CTE_query_definition:
ORDER BY (except when a TOP clause is specified)
OPTION clause with query hints
When a CTE is used in a statement that is part of a batch, the statement before it must be followed by a semicolon.
A query referencing a CTE can be used to define a cursor.
Tables on remote servers can be referenced in the CTE.
When executing a CTE, any hints that reference a CTE may conflict with other hints that are discovered when the CTE accesses its underlying tables, in the same manner as hints that reference views in queries. When this occurs, the query returns an error.
Guidelines for Defining and Using Recursive Common Table Expressions
The following guidelines apply to defining a recursive common table expression:
The recursive CTE definition must contain at least two CTE query definitions, an anchor member and a recursive member. Multiple anchor members and recursive members can be defined; however, all anchor member query definitions must be put before the first recursive member definition. All CTE query definitions are anchor members unless they reference the CTE itself.
Anchor members must be combined by one of these set operators: UNION ALL, UNION, INTERSECT, or EXCEPT. UNION ALL is the only set operator allowed between the last anchor member and first recursive member, and when combining multiple recursive members.
The number of columns in the anchor and recursive members must be the same.
The data type of a column in the recursive member must be the same as the data type of the corresponding column in the anchor member.
The FROM clause of a recursive member must refer only one time to the CTE expression_name.
The following items are not allowed in the CTE_query_definition of a recursive member:
PIVOT (When the database compatibility level is 110 or higher. See Breaking Changes to Database Engine Features in SQL Server 2016.)
LEFT , RIGHT , OUTER JOIN ( INNER JOIN is allowed)
A hint applied to a recursive reference to a CTE inside a CTE_query_definition.
The following guidelines apply to using a recursive common table expression:
All columns returned by the recursive CTE are nullable regardless of the nullability of the columns returned by the participating SELECT statements.
An incorrectly composed recursive CTE may cause an infinite loop. For example, if the recursive member query definition returns the same values for both the parent and child columns, an infinite loop is created. To prevent an infinite loop, you can limit the number of recursion levels allowed for a particular statement by using the MAXRECURSION hint and a value between 0 and 32,767 in the OPTION clause of the INSERT , UPDATE , DELETE , or SELECT statement. This lets you control the execution of the statement until you resolve the code problem that is creating the loop. The server-wide default is 100. When 0 is specified, no limit is applied. Only one MAXRECURSION value can be specified per statement. For more information, see Query Hints (Transact-SQL).
A view that contains a recursive common table expression cannot be used to update data.
Cursors may be defined on queries using CTEs. The CTE is the select_statement argument that defines the result set of the cursor. Only fast forward-only and static (snapshot) cursors are allowed for recursive CTEs. If another cursor type is specified in a recursive CTE, the cursor type is converted to static.
Tables on remote servers may be referenced in the CTE. If the remote server is referenced in the recursive member of the CTE, a spool is created for each remote table so the tables can be repeatedly accessed locally. If it is a CTE query, Index Spool/Lazy Spools is displayed in the query plan and will have the additional WITH STACK predicate. This is one way to confirm proper recursion.
Analytic and aggregate functions in the recursive part of the CTE are applied to the set for the current recursion level and not to the set for the CTE. Functions like ROW_NUMBER operate only on the subset of data passed to them by the current recursion level and not the entire set of data passed to the recursive part of the CTE. For more information, see example K. Using analytical functions in a recursive CTE that follows.
Features and Limitations of Common Table Expressions in Azure Synapse Analytics and Analytics Platform System (PDW)
The current implementation of CTEs in Azure Synapse Analytics and Analytics Platform System (PDW) have the following features and limitations:
A CTE can be specified in a SELECT statement.
A CTE can be specified in a CREATE VIEW statement.
A CTE can be specified in a CREATE TABLE AS SELECT (CTAS) statement.
A CTE can be specified in a CREATE REMOTE TABLE AS SELECT (CRTAS) statement.
A CTE can be specified in a CREATE EXTERNAL TABLE AS SELECT (CETAS) statement.
A remote table can be referenced from a CTE.
An external table can be referenced from a CTE.
Multiple CTE query definitions can be defined in a CTE.
A CTE must be followed by a single SELECT statement. INSERT , UPDATE , DELETE , and MERGE statements are not supported.
A common table expression that includes references to itself (a recursive common table expression) is not supported.
Specifying more than one WITH clause in a CTE is not allowed. For example, if a CTE query definition contains a subquery, that subquery cannot contain a nested WITH clause that defines another CTE.
An ORDER BY clause cannot be used in the CTE_query_definition, except when a TOP clause is specified.
When a CTE is used in a statement that is part of a batch, the statement before it must be followed by a semicolon.
When used in statements prepared by sp_prepare , CTEs will behave the same way as other SELECT statements in PDW. However, if CTEs are used as part of CETAS prepared by sp_prepare , the behavior can defer from SQL Server and other PDW statements because of the way binding is implemented for sp_prepare . If SELECT that references CTE is using a wrong column that does not exist in CTE, the sp_prepare will pass without detecting the error, but the error will be thrown during sp_execute instead.
Examples
A. Creating a simple common table expression
The following example shows the total number of sales orders per year for each sales representative at Adventure Works Cycles.
B. Using a common table expression to limit counts and report averages
The following example shows the average number of sales orders for all years for the sales representatives.
C. Using multiple CTE definitions in a single query
The following example shows how to define more than one CTE in a single query. Notice that a comma is used to separate the CTE query definitions. The FORMAT function, used to display the monetary amounts in a currency format, is available in SQL Server 2012 and higher.
Here is a partial result set.
D. Using a recursive common table expression to display multiple levels of recursion
The following example shows the hierarchical list of managers and the employees who report to them. The example begins by creating and populating the dbo.MyEmployees table.
Using a recursive common table expression to display two levels of recursion
The following example shows managers and the employees reporting to them. The number of levels returned is limited to two.
Using a recursive common table expression to display a hierarchical list
The following example adds the names of the manager and employees, and their respective titles. The hierarchy of managers and employees is additionally emphasized by indenting each level.
Using MAXRECURSION to cancel a statement
MAXRECURSION can be used to prevent a poorly formed recursive CTE from entering into an infinite loop. The following example intentionally creates an infinite loop and uses the MAXRECURSION hint to limit the number of recursion levels to two.
After the coding error is corrected, MAXRECURSION is no longer required. The following example shows the corrected code.
E. Using a common table expression to selectively step through a recursive relationship in a SELECT statement
The following example shows the hierarchy of product assemblies and components that are required to build the bicycle for ProductAssemblyID = 800 .
F. Using a recursive CTE in an UPDATE statement
The following example updates the PerAssemblyQty value for all parts that are used to build the product 'Road-550-W Yellow, 44' (ProductAssemblyID``800 ). The common table expression returns a hierarchical list of parts that are used to build ProductAssemblyID 800 and the components that are used to create those parts, and so on. Only the rows returned by the common table expression are modified.
H. Using multiple anchor and recursive members
The following example uses multiple anchor and recursive members to return all the ancestors of a specified person. A table is created and values inserted to establish the family genealogy returned by the recursive CTE.
I. Using analytical functions in a recursive CTE
The following example shows a pitfall that can occur when using an analytical or aggregate function in the recursive part of a CTE.
The following results are the expected results for the query.
The following results are the actual results for the query.
N returns 1 for each pass of the recursive part of the CTE because only the subset of data for that recursion level is passed to ROWNUMBER . For each of the iterations of the recursive part of the query, only one row is passed to ROWNUMBER .
Examples: Azure Synapse Analytics and Analytics Platform System (PDW)
J. Using a common table expression within a CTAS statement
The following example creates a new table containing the total number of sales orders per year for each sales representative at Adventure Works Cycles.
K. Using a common table expression within a CETAS statement
The following example creates a new external table containing the total number of sales orders per year for each sales representative at Adventure Works Cycles.
L. Using multiple comma separated CTEs in a statement
The following example demonstrates including two CTEs in a single statement. The CTEs cannot be nested (no recursion).
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