How get information from multiple tables using cursor?

问题:

I have a query, that returns multiple tables, something like that:

SELECT TableName, DatabaseName +'.'+ TableName, ColumnName FROM DBC.Columns WHERE ColumnName = 'id' 

And I need to loop through these tables by looking to the information stored in these tables, in order to get only specific tables.

I tried something like code below, using 'LOOP' and cursor, but it says that Query is invalid (code have been taken from here):

DECLARE cursor_Tables CURSOR FOR          SELECT DatabaseName || '.' || TableName     FROM   DBC.Columns     WHERE  ColumnName  ='id';   OPEN cursor_Tables;      label1:      LOOP             FETCH  cursor_Tables into tbName;         IF (SQLSTATE ='02000') THEN             LEAVE label1;         END IF;          CASE WHEN (               SELECT COUNT(*)             FROM prd3_db_tmd.K_PTY_NK01             WHERE id = 0 ) > 0              THEN tbName         END      END LOOP label1; CLOSE cursor_Tables; END; 

How can I actually deal with this problem? Do I need to use procedure in addition? DBMS is Teradata

回答1:

You need a Stored Procedure because this is the only place where you can use a cursor in Teradata.

REPLACE PROCEDURE testproc() DYNAMIC RESULT SETS 1 BEGIN    DECLARE tbName VARCHAR(257);    DECLARE SqlStr VARCHAR(500);     -- temporary table to store the result set    CREATE VOLATILE TABLE _vt_(tbName VARCHAR(257)) ON COMMIT PRESERVE ROWS;     -- your existing query to return the table name    -- Better use ColumnsV instead of Columns    FOR cursor_Tables AS            SELECT DatabaseName || '.' || TABLENAME AS tbName        FROM   DBC.ColumnsV        WHERE  ColumnName  ='id'    DO -- prepare the dynamic SQL ...       SET SqlStr =           'insert into _vt_           select ''' || cursor_tables.tbName || '''            from ' || cursor_tables.tbName || '           where id = 0            having count(*) > 0;           ';       -- ... and run it       EXECUTE IMMEDIATE SqlStr;    END FOR;     BEGIN -- return the result set       DECLARE resultset CURSOR WITH RETURN ONLY FOR S1;       SET SqlStr = 'SELECT * FROM _vt_;';       PREPARE S1 FROM SqlStr;       OPEN resultset;    END;     DROP TABLE vt; END; 

回答2:

If this is SQL Server you can check following SQL cursor, I edited the cursor declaration and the code within Although they may differ from your requirement, I think you can modify easily

declare @sql nvarchar(max) declare @tablename nvarchar(100)  DECLARE cursor_Tables CURSOR FOR          SELECT s.name + '.' + o.name          --s.name [schema], o.name [table]     FROM   sys.Columns c     inner join sys.objects o on c.object_id = o.object_id     inner join sys.schemas s on s.schema_id = o.schema_id     WHERE  c.Name  ='id' and o.type = 'U'  /* SELECT TableName, DatabaseName +'.'+ TableName, ColumnName FROM DBC.Columns WHERE ColumnName = 'id' */ OPEN cursor_Tables;   FETCH NEXT FROM cursor_Tables INTO @tablename  WHILE @@FETCH_STATUS = 0 BEGIN  --  print @tablename set @sql = 'select case when count(*) > 0 then ''' + @tablename + ''' else '''' end from ' + @tablename exec sp_executesql @sql   FETCH NEXT FROM cursor_Tables INTO @tablename END  CLOSE cursor_Tables; DEALLOCATE cursor_Tables; 

回答3:

On SQL Server, sp_MsForEachTable undocumented stored procedure can be used instead of a loop structure like a cursor

Please check the below SQL command

EXEC sp_MSForEachTable 'IF EXISTS(select * from sys.columns where name = ''Id'' and object_id = object_id(''?''))SELECT ''?'', COUNT(*) FROM ?' 

The syntax may be difficult if you are using the sp_msforeachtable or sp_msforeachdb, but you can find samples on the web

回答4:

You could create a variable to hold the number of rows and set it equal to the count:

DECLARE @count INT  SELECT @count = COUNT(*) FROM prd3_db_tmd.K_PTY_NK01 WHERE id = 0 

Then use an if statement to select the table if it has rows that meet your criteria:

IF @count > 0 BEGIN     SELECT tbName END 

---

Also as a side note without having SELECT in front of your CASE statement the syntax is invalid, you may want to try it with just adding SELECT in front of CASE if you don't like the way mentioned above

回答5:

You need to use dynamic SQL. If you need to see the info on the table, you can create a synonym.

    CURSOR  cursor_Tables is         SELECT DatabaseName || '.' || TableName AS tbName          FROM   DBC.Columns         WHERE  ColumnName  ='id';   begin       FOR R IN cursor_Tables     LOOP          execute immediate 'CREATE OR REPLACE SYNONYM your_synonym FOR '|| R.tbName ;      select *     from your_synonym;       END LOOP; END; 

Or if you want you can create a view.

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