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Merge pull request #45 from oracle-sql-features/issue-41
add row_limiting_clause with examples
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‎features/query-row-limits-and-offsets.adoc

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= Native SQL Support for Query Row Limits and Row Offsets
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:database-version: 12.1.0.1
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:database-category: SQL
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[[feature_summary]]
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Many SQL queries benefit from limiting the result set to the top-_n_, or first _n_ rows.
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Before Oracle 12c Release 1 limiting the result set required the use of sub-selects and the rownum pseudo-column and inline views as you can see in the first examples. When Oracle Database 12c introduced support for the https://docs.oracle.com/en/database/oracle/oracle-database/23/sqlrf/SELECT.html#GUID-CFA006CA-6FF1-4972-821E-6996142A51C6__BABBADDD[row limiting clause] selecting the first _n_ rows from a result set, or paging through a result set were significantly improved.
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[source,sql]
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[subs="verbatim"]
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----
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/*
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Displaying the top 10 customers by sales using the pre-
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Oracle Database 12c syntax
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*/
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SELECT
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*
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FROM
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(
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SELECT
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SUM(quantity_sold * amount_sold) AS revenue,
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cust_id
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FROM
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sales
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GROUP BY
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cust_id
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ORDER BY
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SUM(quantity_sold * amount_sold) DESC
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)
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WHERE
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ROWNUM < 11;
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/*
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Iterating over the result set, displaying 10 customers each using the pre-
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Oracle Database 12c syntax. You typically find this type of query in reports
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where batches of rows are displayed on screen. The following query retrieves
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rows n to m (say, 10 to 20) from the result set.
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*/
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VAR n NUMBER
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VAR m NUMBER
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BEGIN
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:n := 10;
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:m := 20;
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END;
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/
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SELECT
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*
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FROM
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(
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SELECT
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a.*,
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ROWNUM rn
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FROM
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(
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SELECT
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SUM(quantity_sold * amount_sold) AS revenue,
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cust_id
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FROM
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sales
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GROUP BY
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cust_id
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ORDER BY
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SUM(quantity_sold * amount_sold) DESC
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) a
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WHERE
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ROWNUM <= :m
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)
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WHERE
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rn >= :n;
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/*
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The same queries, however this time using the new syntax, available from
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Oracle Database 12c Release 1 onwards
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*/
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SELECT
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SUM(quantity_sold * amount_sold) AS revenue,
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cust_id
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FROM
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sales
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GROUP BY
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cust_id
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ORDER BY
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SUM(quantity_sold * amount_sold) DESC
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FETCH FIRST 10 ROWS ONLY;
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SELECT
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SUM(quantity_sold * amount_sold) AS revenue,
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cust_id
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FROM
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sales
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GROUP BY
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cust_id
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ORDER BY
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SUM(quantity_sold * amount_sold) DESC
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OFFSET 9 ROWS FETCH NEXT 11 ROWS ONLY;
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----
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.Result
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[source,sql]
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[subs="verbatim"]
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----
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SQL> /*
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SQL> Displaying the top 10 customers by sales using the pre-
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SQL> Oracle Database 12c syntax
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SQL>*/
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SQL> SELECT
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2 *
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3 FROM
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4 (
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5 SELECT
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6 SUM(quantity_sold * amount_sold) AS revenue,
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7 cust_id
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8 FROM
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9 sales
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10 GROUP BY
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11 cust_id
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12 ORDER BY
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13 SUM(quantity_sold * amount_sold) DESC
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14 )
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15 WHERE
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16 ROWNUM < 11;
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REVENUE CUST_ID
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____________ __________
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103412,66 11407
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99578,09 10747
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98585,96 42167
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98006,16 4974
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97573,55 12783
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97010,48 6395
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94862,61 2994
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94819,41 429
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94786,13 1743
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93644,32 4759
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10 rows selected.
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SQL> /*
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SQL> Iterating over the result set, displaying 10 customers each using the pre-
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SQL> Oracle Database 12c syntax. You typically find this type of query in reports
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SQL> where batches of rows are displayed on screen. The following query retrieves
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SQL> rows n to m (say, 10 to 20) from the result set.
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SQL>*/
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SQL> VAR n NUMBER
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SQL> VAR m NUMBER
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SQL> BEGIN
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2 :n := 10;
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3 :m := 20;
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4 END;
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5 /
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PL/SQL procedure successfully completed.
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SQL> SELECT
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2 *
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3 FROM
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4 (
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5 SELECT
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6 a.*,
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7 ROWNUM rn
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8 FROM
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9 (
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10 SELECT
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11 SUM(quantity_sold * amount_sold) AS revenue,
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12 cust_id
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13 FROM
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14 sales
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15 GROUP BY
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16 cust_id
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17 ORDER BY
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18 SUM(quantity_sold * amount_sold) DESC
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19 ) a
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20 WHERE
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21 ROWNUM <= :m
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22 )
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23 WHERE
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24 rn >= :n;
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REVENUE CUST_ID RN
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___________ __________ _____
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93644,32 4759 10
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92671,18 9038 11
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90908,24 4090 12
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89018,86 7178 13
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86440,63 6279 14
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85278,17 11702 15
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85200,78 6265 16
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84261,92 25939 17
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83833,3 7994 18
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83461,16 20966 19
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81970,44 3453 20
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11 rows selected.
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SQL> /*
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SQL> The same queries, however this time using the new syntax, available from
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SQL> Oracle Database 12c Release 1 onwards
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SQL>*/
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SQL> SELECT
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2 SUM(quantity_sold * amount_sold) AS revenue,
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3 cust_id
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4 FROM
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5 sales
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6 GROUP BY
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7 cust_id
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8 ORDER BY
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9 SUM(quantity_sold * amount_sold) DESC
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10 FETCH FIRST 10 ROWS ONLY;
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REVENUE CUST_ID
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____________ __________
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103412,66 11407
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99578,09 10747
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98585,96 42167
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98006,16 4974
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97573,55 12783
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97010,48 6395
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94862,61 2994
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94819,41 429
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94786,13 1743
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93644,32 4759
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10 rows selected.
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SQL> SELECT
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2 SUM(quantity_sold * amount_sold) AS revenue,
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3 cust_id
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4 FROM
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5 sales
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6 GROUP BY
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7 cust_id
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8 ORDER BY
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9 SUM(quantity_sold * amount_sold) DESC
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10 OFFSET 9 ROWS FETCH NEXT 11 ROWS ONLY;
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REVENUE CUST_ID
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___________ __________
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93644,32 4759
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92671,18 9038
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90908,24 4090
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89018,86 7178
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86440,63 6279
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85278,17 11702
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85200,78 6265
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84261,92 25939
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83833,3 7994
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83461,16 20966
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81970,44 3453
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11 rows selected.
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----
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== Benefits
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The FETCH FIRST and OFFSET clauses provides native SQL language support to limit the number of rows returned and to specify a starting row for the return set.
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Many queries need to limit the number of rows returned or offset the starting row of the results. For example, top-N queries sort their result set and then return only the first n rows. FETCH FIRST and OFFSET simplify syntax and comply with the ANSI SQL standard.
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== Further information
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* Availability: All Offerings
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* https://docs.oracle.com/en/database/oracle/oracle-database/23/sqlrf/SELECT.html#GUID-CFA006CA-6FF1-4972-821E-6996142A51C6__BABBADDD[SQL Language Reference]
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* https://blogs.oracle.com/sql/post/how-to-select-the-top-n-rows-per-group-with-sql-in-oracle-database[Blog post with additional examples] covering many aspects of top-_n_ queries, including analytic functions

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