How to return information about SQL Server linked servers

Summary

SQL Server 7.0 and later versions of SQL Server allow the addition of external data sources as linked servers. This feature provides access to distributed, heterogeneous queries against OLE DB data sources. When connecting to SQL Server, a cursor listing available tables can be returned with the SQLTables(file_handle) command. The SQLTables() command, however, does not return information about tables available in Linked Servers. Information regarding linked servers is returned by three different SQL Server stored procedures. This article describes how to use those stored procedures to return information regarding tables available from Linked Servers.

More Information

SQL Server linked servers are defined as mappings against a specific linked server. Therefore, a linked server may not necessarily be part of a SQL Server database.

SQL Server linked servers are defined using the sp_addlinkedserver system stored procedure. The linked server definition contains all the information needed to locate the OLE DB data source. Remote tables from the linked server name can then be referenced in two ways:
  • The linked server name can be used as the server name in a four-part name used as a table or view reference in a Transact-SQL statement. The other three parts of the name reference an object in the linked server that is exposed as a rowset.
  • The linked server name can be used as an input parameter to an OPENQUERY function. OPENQUERY sends the OLE DB provider a command to execute. The returned rowset can then be used as a table or view reference in a Transact-SQL statement.
The SQL Server stored procedures used to return information about linked servers are:
  • sp_linkedservers
  • sp_tables_ex
  • sp_columns_ex
Each of the SQL Server stored procedures return a different level of detail related to linked servers.

sp_linkedservers returns the following columns:



SRV_NAME Name of the linked server.

SRV_PROVIDERNAME Friendly name of the OLE DB provider managing
access to the specified linked server.

SRV_PRODUCT Product name of the linked server.

SRV_DATASOURCE OLE DB datasource property corresponding to the
specified linked server.

SRV_PROVIDERSTRING OLE DB provider string property corresponding to
the specified linked server.

SRV_LOCATION OLE DB location property corresponding to the
specified linked server.

SRV_CAT OLE DB catalog property corresponding to the
specified linked server.
sp_tables_ex returns information on the tables from the specified linked server. This stored procedure accepts the following arguments:


TABLE_SERVER Name of the linked server for which to return table
information, with no default.

TABLE_NAME Name of the table for which to return data type
information, with a default of NULL.

TABLE_SCHEMA Is the table schema, with a default of NULL.

TABLE_CATALOG Catalog Name of the database in which the specified
table resides, with a default of NULL.

TABLE_TYPE Type of the table to return, with a default of NULL.

sp_tables_ex returns the following columns:


TABLE_CAT Table qualifier name. (May be null)

TABLE_SCHEM Table or view owner name.

TABLE_NAME Table name.

TABLE_TYPE Table, system table, or view.

REMARKS SQL Server does not return a value for this column.

sp_columns_ex returns column information, one row per column, for the given linked server table(s). sp_columns_ex returns column information only for the given column if column is specified. This stored procedure accepts the following arguments:


TABLE_SERVER Name of the linked server for which to return column
information, with no default.

TABLE_NAME Name of the table for which to return column
information, with a default of NULL.

TABLE_SCHEMA Name of the table for which to return column
information, with a default of NULL.

TABLE_CATALOG Catalog name of the table for which to return column
information, with a default of NULL.

COLUMN Name of the database column for which to provide
information, with a default of NULL.

ODBCVer The version of ODBC being used. ODBCVer is an
integer, with a default value of 2, indicating ODBC
Version 2. Valid values are 2 or 3.

sp_columns_ex returns the following columns:


TABLE_CAT Table or view qualifier name. (May be null)

TABLE_SCHEM Table or view owner name.

TABLE_NAME Table or view name.

COLUMN_NAME Column name, for each column of the specified table.

DATA_TYPE Integer value corresponding to ODBC type indicators.

TYPE_NAME String representing a data type.

COLUMN_SIZE Number of significant digits.

BUFFER_LENGTH Transfer size of the data.

DECIMAL_DIGITS Number of digits to the right of the decimal point.

NUM_PREC_RADIX Is the base for numeric data types.

NULLABLE Specifies nullability.

REMARKS This field always returns NULL.

COLUMN_DEF Default value of the column.

SQL_DATA_TYPE Value of the SQL data type as it appears in the TYPE
field of the descriptor.

SQL_DATETIME_SUB Subtype code for datetime and SQL-92 interval data
types. For other data types, this column returns
NULL.

CHAR_OCTET_LENGTH Maximum length in bytes of a character or integer
data type column. For all other data types, this
column returns NULL.

ORDINAL_POSITION Ordinal position of the column in the table.
IS_NULLABLE Nullability of the column in the table. This column
returns a zero-length string if nullability is
unknown. The value returned for this column is
different from the value returned for the NULLABLE
column.

SS_DATA_TYPE SQL Server data type, used by Open Data Services
extended stored procedures.

  1. Open the ODBC Administrator and create an ODBC DataSource named "LINKSRV".

    Note Because linked servers are listed under the SQL Server Security tab, it is not necessary to set a default database when creating the DS.
  2. Create a program file named "SP_tables.prg" using the following code:


       * NOTE: This code is based on the following assumptions:
    * A DSN named LINKSRV pointing to a SQL Server data source exists.
    * A Linked Server has been defined in SQL Server.
    Connect_String="'LINKSRV','SA',''"
    * Connect to SQL Server
    gnConnHandle=SQLCONNECT(&Connect_String)
    IF gnConnHandle>0
    * Create a command string to pass to SQL Server via SQLExec.
    CREATE CURSOR SQLSRVDAT (Server c(128),Table_Name c(128), ;
    Column_name c(128))
    * Create a command string to pass to SQL Server via SQLExec.
    SQLCommand="sp_linkedservers"
    * Return a cursor with the names of available linked servers.
    QRYVal=SQLExec(gnConnHandle,SQLCommand,'SQLSvr')
    IF QRYVal>0
    SELECT SQLSvr
    DO WHILE !EOF()
    * Build a command to pass to SQL Server to return a cursor
    * of tables available from the linked server.
    SQLCommand="sp_tables_ex '"+ALLTRIM(SQLSvr.SRV_NAME)+"'"
    * Return a cursor with the names of tables available from
    * the specified linked server.
    tables_val=SQLExec(gnConnHandle,SQLCommand,'SQLTabs')
    IF tables_val>0
    SELECT sqltabs
    LOCA
    DO WHILE !EOF()
    * Build a command to pass to SQL Server to return a
    * cursor with the fields from the specified
    * table of the defined linked server.
    SQLCommand="sp_columns_ex '"+ALLTRIM(SQLSvr.SRV_NAME) + ;
    "','"+ALLTRIM(sqltabs.Table_Name)+"'"
    * Return a cursor with the names of the columns
    * in the specified table of the linked server.
    ColVal=SQLExec(gnConnHandle,SQLCommand,'sqlcols')
    IF ColVal>0
    SELECT sqlcols
    DO WHILE !EOF()
    INSERT INTO SQLSRVDAT VALUES ;
    (SQLSvr.SRV_NAME,sqltabs.Table_Name, ;
    sqlcols.Column_name)
    SELECT sqlcols
    SKIP
    ENDDO
    ENDIF
    SELECT sqltabs
    SKIP
    ENDDO
    ENDIF
    SELECT SQLSvr
    SKIP
    ENDDO
    ELSE
    =AERROR(L_Server)
    =MESSAGEBOX(L_Server[1,2]+CHR(13)+L_Server[1,3],32,'Query Failed')
    ENDIF
    =SQLDISCONN(gnConnHandle)
    SELECT SQLSRVDAT
    IF RECCOUNT()=0
    =MESSAGEBOX('No Linked Servers Detected',64,'Linked Servers')
    ELSE
    CALCULATE MAX(LEN(ALLTRIM(SQLSRVDAT.column_Name))) TO max_wide
    BROW TITLE "Linked Server Information" WIDTH max_wide
    ENDIF
    ELSE
    =AERROR(s_failed)
    * The linked server doesn't exist, so display a message.
    =MESSAGEBOX(s_failed[1,2]+CHR(13)+ ;
    IIF(!ISNULL(s_failed[1,3]),s_failed[1,3],"null"),32,'Failed')
    ENDIF
    RETURN
    * End Code.
  3. In the Command window, type the following:

    DO SP_TABLES

References

Transact - SQL Reference Help; search on: "sp_addlinkedserver"; "sp_tables_ex"; "sp_linkedservers"; "sp_columns_ex"

Propriétés

ID d'article : 203638 - Dernière mise à jour : 20 juin 2008 - Révision : 1

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