SELECT Statement for Schema Queries (Windows)

SELECT Statement for Schema Queries

SELECT Statement for Schema Queries

Schema data queries use the SELECT statement with a syntax similar to that for data queries. The difference is the use of a special class called "meta_class", which identifies the query as a schema query.

The following example requests all class definitions that are within the current namespace.

SELECT * FROM meta_class

Schema queries only support "*". To narrow the scope of the definitions returned, a provider can add a WHERE clause that specifies a particular class.

The following example shows how to add a WHERE clause to specify a particular class.

SELECT * FROM meta_class WHERE __this ISA "Win32_LogicalDisk"

The special property called __this identifies the target class for a schema query. Note that the ISA operator must be used with the __this property to request definitions for the subclasses of the target class. The preceding query returns the definition for the Win32_LogicalDisk class and definitions for all of its subclasses.

The following example shows how to request a class definition for a single class by using the __Class system property.

SELECT * FROM meta_class WHERE __Class = "Win32_LogicalDisk"

This query is equivalent to calling the IWbemServices::GetObject or the IWbemServices::GetObjectAsync method with the object path parameter set to "Win32_LogicalDisk".

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Build date: 3/9/2012

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Retrieve all child classes of a top level WMI class

urkec | Edit | Show History

__Dynasty WMI system property holds the name of the top-level class from which a class is derived. You can use it to retrieve all classes in a namespace derived from a top level class, including that class. Here is a VBScript sample:

' VBScript source code

' Retrieve immediate child classes for Cim_DataFile

Set objWmi = GetObject ("winmgmts:root\cimv2")

Set colClasses = objWmi.ExecQuery _ 
    ("Select * From meta_class " _
    &;; "Where __Dynasty = 'Win32_CurrentTime'")

For Each objClass In colClasses 
    WScript.Echo objClass.SystemProperties_("__Class")
Next

The output is:

Win32_CurrentTime
Win32_LocalTime
Win32_UTCTime

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Retrieve immediate child classes for a WMI class using VBScript

urkec | Edit | Show History

The following query allows you to retrieve immediate child classes for a WMI class:

Select * From meta_class Where __SuperClass = "Cim_DataFile"

This VBScript sample gets all the immediate child classes for Cim_DataFile class:

' VBScript source code

' Retrieve immediate child classes for Cim_DataFile

Set objWmi = GetObject ("winmgmts:root\cimv2")

Set colClasses = objWmi.ExecQuery _ 
    ("Select * From meta_class " _
    &;; "Where __Superclass = 'Cim_DataFile'")

For Each objClass In colClasses 
    WScript.Echo objClass.SystemProperties_("__Class")
Next

The script produces the following output (Windows Xp SP3):

Win32_ShortcutFile
Win32_CodecFile
Win32_NTEventlogFile
Win32_PageFile

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Retrieve top level classes using VBScript

urkec | Edit | Show History

For all the top level classes in a WMI namespece the __Superclass system property is Null, so it is possible to retrieve them using this query:

Select * From meta_class Where __Superclass Is Null

Here is a VBScript sample that retrieves all the top classes in Root\Cimv2 WMI namespace:

' VBScript source code

' Retrieve top level classes in root\cimv2

Set objWmi = GetObject ("winmgmts:root\cimv2")

Set colClasses = objWmi.ExecQuery _
    ("Select * From meta_class Where __Superclass Is Null")

For Each objClass In colClasses
    WScript.Echo objClass.SystemProperties_("__Class")
Next

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