TOP and SAMPLE

The TOP and SAMPLE clauses of Teradata are migrated to LIMIT in GaussDB(DWS).

TOP

The DSC also supports migration of TOP statements with dynamic parameters.

  • For TOP clauses containing WITH TIES, the ORDER BY clause is also required. Otherwise, the tool will not migrate the statement and copy it as it is.
  • When using TOP with dynamic parameters:
    • The input dynamic parameters should be in the following form:
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       TOP :<parameter_name>

      The following characters are valid for dynamic parameters: a-z, A-Z, 0-9 and "_".

Input: SELECT .. TOP

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SELECT TOP 1 c1, COUNT (*) cnt 
  FROM tab1 
 GROUP BY c1 
 ORDER BY cnt;

Output

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SELECT c1, COUNT( * ) cnt
  FROM tab1
 GROUP BY c1
 ORDER BY cnt 
 LIMIT 1;

Input: SELECT .. TOP PERCENT

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SELECT TOP 10 PERCENT c1, c2 
  FROM employee
 WHERE ...
 ORDER BY c2 DESC;

Output

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WITH top_percent AS (
      SELECT c1, c2
        FROM employee
       WHERE ...
       ORDER BY c2 DESC
                    ) 
SELECT *
  FROM top_percent 
 LIMIT (SELECT CEIL(COUNT( * ) * 10 / 100)
          FROM top_percent);

Input: SELECT .. TOP with dynamic parameters

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SELECT
           TOP :Limit WITH TIES c1
          ,SUM (c2) sc2
     FROM
          tab1
     WHERE
          c3 > 10
     GROUP BY
          c1
     ORDER BY
          c1
;

Output

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WITH top_ties AS (
     SELECT
                c1
               ,SUM (c2) sc2
               ,rank (
               ) OVER( ORDER BY c1 ) AS TOP_RNK
          FROM
               tab1
          WHERE
               c3 > 10
          GROUP BY
               c1
) SELECT
          c1
          ,sc2
     FROM
          top_ties
     WHERE
          TOP_RNK <= :Limit
     ORDER BY
          TOP_RNK
;

Input: SELECT .. TOP with dynamic parameters and with TIES

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 SELECT
           TOP :Limit WITH TIES Customer_ID
   FROM
          Customer_t
   ORDER BY
          Customer_ID
;

Output

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WITH top_ties AS (
     SELECT
               Customer_ID
               ,rank (
               ) OVER( order by Customer_id) AS TOP_RNK
          FROM
               Customer_t
) SELECT
          Customer_ID
     FROM
          top_ties
     WHERE
          TOP_RNK <= :Limit
     ORDER BY
          TOP_RNK
;

Input: SELECT .. TOP PERCENT with dynamic parameters

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SELECT
          TOP :Input_Limit PERCENT WITH TIES c1
          ,SUM (c2) sc2
     FROM
          tab1
     GROUP BY
          c1
     ORDER BY
          c1
;

Output

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WITH top_percent_ties AS (
     SELECT
               c1
               ,SUM (c2) sc2
               ,rank (
               ) OVER( ORDER BY c1 ) AS TOP_RNK
          FROM
               tab1
          GROUP BY
               c1
) SELECT
          c1
          ,sc2
     FROM
          top_percent_ties
     WHERE
          TOP_RNK <= (
               SELECT
                         CEIL(COUNT( * ) * :Input_Limit / 100)
                    FROM
                         top_percent_ties
          )
     ORDER BY
          TOP_RNK
;

SAMPLE

The tool only supports single positive integers in the SAMPLE clause.

Input: SELECT .. SAMPLE

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SELECT c1, c2, c3 
  FROM tab1 
 WHERE c1 > 1000
SAMPLE 1;

Output

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SELECT c1, c2, c3
  FROM tab1
 WHERE c1 > 1000
 LIMIT 1;
Parent topic: SELECT