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Subscription Rule SQL Action Syntax

  • Article

A SQL action is used to manipulate message metadata after a message has been selected by a filter of a subscription rule. It's a text expression that leans on a subset of the SQL-92 standard. Action expressions are used with the sqlExpression element of the 'action' property of a Service Bus Rule in an Azure Resource Manager template, or the Azure CLI az servicebus topic subscription rule create command's --action-sql-expression argument, and several SDK functions that allow managing subscription rules.

<statements> ::=
    <statement> [, ...n]  
  

<statement> ::=
    <action> [;]
    Remarks
    -------
    Semicolon is optional.  
  

<action> ::=
    SET <property> = <expression>
    REMOVE <property>  

<expression> ::=
    <constant>
    | <function>
    | <property>
    | <expression> { + | - | * | / | % } <expression>
    | { + | - } <expression>
    | ( <expression> )

<property> := 
    [<scope> .] <property_name>

Arguments

  • <scope> is an optional string indicating the scope of the <property_name>. Valid values are sys or user.
    • The sys value indicates system scope where <property_name> is any of the properties on the Service Bus message as described in Messages, payloads, and serialization.
    • The user value indicates user scope where <property_name> is a key of the custom properties that you can set on the message when sending to Service Bus.
    • The user scope is the default scope if <scope> isn't specified.

Remarks

An attempt to access a nonexistent system property is an error, while an attempt to access a nonexistent user property isn't an error. Instead, a nonexistent user property is internally evaluated as an unknown value. An unknown value is treated specially during operator evaluation.

property_name

<property_name> ::=  
     <identifier>  
     | <delimited_identifier>  
  
<identifier> ::=  
     <regular_identifier> | <quoted_identifier> | <delimited_identifier>

Arguments

<regular_identifier> is a string represented by the following regular expression:

[[:IsLetter:]][_[:IsLetter:][:IsDigit:]]*

It means any string that starts with a letter and is followed by one or more underscore/letter/digit.

[:IsLetter:] means any Unicode character that is categorized as a Unicode letter. System.Char.IsLetter(c) returns true if c is a Unicode letter.

[:IsDigit:] means any Unicode character that is categorized as a decimal digit. System.Char.IsDigit(c) returns true if c is a Unicode digit.

A <regular_identifier> can't be a reserved keyword.

<delimited_identifier> is any string that is enclosed with left/right square brackets ([]). A right square bracket is represented as two right square brackets. The following are examples of <delimited_identifier>:

[Property With Space]  
[HR-EmployeeID]

<quoted_identifier> is any string that is enclosed with double quotation marks. A double quotation mark in identifier is represented as two double quotation marks. It isn't recommended to use quoted identifiers because it can easily be confused with a string constant. Use a delimited identifier if possible. Here's an example of <quoted_identifier>:

"Contoso & Northwind"

Pattern

<pattern> ::=  
      <expression>

Remarks

<pattern> must be an expression that is evaluated as a string. It's used as a pattern for the LIKE operator. It can contain the following wildcard characters:

  • %: Any string of zero or more characters.
  • _: Any single character.

escape_char

<escape_char> ::=  
      <expression>

Remarks

<escape_char> must be an expression that is evaluated as a string of length 1. It's used as an escape character for the LIKE operator.

For example, property LIKE 'ABC\%' ESCAPE '\' matches ABC% rather than a string that starts with ABC.

Constant

<constant> ::=  
      <integer_constant> | <decimal_constant> | <approximate_number_constant> | <boolean_constant> | NULL

Arguments

  • <integer_constant> is a string of numbers that aren't enclosed in quotation marks and don't contain decimal points. The values are stored as System.Int64 internally, and follow the same range.

    The following are examples of long constants:

    1894  
2

  • <decimal_constant> is a string of numbers that aren't enclosed in quotation marks, and contain a decimal point. The values are stored as System.Double internally, and follow the same range/precision.

    In a future version, this number might be stored in a different data type to support exact number semantics, so you shouldn't rely on the fact the underlying data type is System.Double for <decimal_constant>.

    The following are examples of decimal constants:

    1894.1204  
2.0

  • <approximate_number_constant> is a number written in scientific notation. The values are stored as System.Double internally, and follow the same range/precision. The following are examples of approximate number constants:

    101.5E5  
0.5E-2

boolean_constant

<boolean_constant> :=  
      TRUE | FALSE

Remarks

Boolean constants are represented by the keywords TRUE or FALSE. The values are stored as System.Boolean.

string_constant

<string_constant>

Remarks

String constants are enclosed in single quotation marks and include any valid Unicode characters. A single quotation mark embedded in a string constant is represented as two single quotation marks.

Function

<function> :=  
      newid() |  
      property(name) | p(name)

Currently, newid() and property(name) are the only functions supported.

Remarks

  • The newid() function returns a System.Guid generated by the System.Guid.NewGuid() method.
  • The property(name) function returns the value of the property referenced by name. The name value can be any valid expression that returns a string value.

Examples

For examples, see Service Bus filter examples.

Considerations

  • SET is used to create a new property or update the value of an existing property.
  • REMOVE is used to remove a user property. Only user properties can be removed, not system properties.
  • SET performs implicit conversion if possible when the expression type and the existing property type are different.
  • Action fails if nonexistent system properties were referenced.
  • Action doesn't fail if nonexistent user properties were referenced.
  • A nonexistent user property is evaluated as "Unknown" internally, following the same semantics as SQLRuleFilter when evaluating operators.

Important points

Here are a few important points:

  • Only properties on a message can be modified.
  • All user properties can be modified.
  • All publicly updatable system properties can also be modified, like ReplyTo and CorrelationId, but we recommend that you don't alter system properties as part of a rule action. It's still allowed for backward compatibility reasons.
  • When setting properties, only numeric, Boolean, and string literals are allowed. A string literal in turn is converted to a type based on the property being modified. If the property being set doesn't already exist, there's no type conversion from string. If the property being modified already exists and its value is one of these types Guid, DateTimeOffset, TimeSpan, Uri, DateTime, then the string literal is converted to that type and set as the property value. To be more specific, the action tries to convert the string literal to the type of property. If it's successful, the property is set. Otherwise, the rule action evaluation throws an exception and the message is dead-lettered.

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