How to return information about SQL Server linked servers

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This article was previously published under Q203638

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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.

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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.

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.

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.