How to use cursors to extract data from multiple tables

Question

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

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

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

I tried the code below, using 'LOOP' and a cursor, but it says Invalid query (code comes 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 should I solve this problem? Do I need to use additional stored procedures? The DBMS is Teradata.

Answer 1

If this is SQL Server you can look at the following SQL Cursors, I edited the cursor declaration and the code in it Although they may be different from your requirements, I think you can modify them 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;

Answer 2

You need a stored procedure because this is the only place where cursors can be used in Teradata.

REPLACE PROCEDURE testproc()
DYNAMIC RESULT SETS 1
BEGIN
   DECLARE tbName VARCHAR(257);
   DECLARE SqlStr VARCHAR(500);

   -- 临时表来存储结果集
   CREATE VOLATILE TABLE _vt_(tbName VARCHAR(257)) ON COMMIT PRESERVE ROWS;

   -- 您现有的查询来返回表名
   -- 最好使用ColumnsV而不是Columns
   FOR cursor_Tables AS    
       SELECT DatabaseName || '.' || TABLENAME AS tbName
       FROM   DBC.ColumnsV
       WHERE  ColumnName  ='id'
   DO -- 准备动态SQL ...
      SET SqlStr = 
         'insert into _vt_
          select ''' || cursor_tables.tbName || ''' 
          from ' || cursor_tables.tbName || '
          where id = 0 
          having count(*) > 0;
          ';
      -- ... 并执行它
      EXECUTE IMMEDIATE SqlStr;
   END FOR;

   BEGIN -- 返回结果集
      DECLARE resultset CURSOR WITH RETURN ONLY FOR S1;
      SET SqlStr = 'SELECT * FROM _vt_;';
      PREPARE S1 FROM SqlStr;
      OPEN resultset;
   END;

   DROP TABLE vt;
END;