Listagg can be combined with the filter and over clauses: LISTAGG(…) WITHIN GROUP(…) ![]() GROUP BY g Combining listagg with filter and over , LISTAGG(val, ',') WITHIN GROUP (ORDER BY o) list The example uses min(o) to keep the first occurrence in case one value appears multiple times. The columns g and o represent the group by and order by keys respectively. The following example demonstrates this approach. This works for databases not supporting distinct in listagg, and also allows to keep a particular occurrence if duplicates exist. ![]() 9ĭistinct can be implemented manually by removing duplicates before aggregation-e.g. If the order by clause places one occurrence at the beginning and the other at the end, it is unspecified at which place the value appears in the result. The standard does not specify which of the duplicate elements is removed. Note that the elimination of duplicates is subject to the collation in effect. If distinct is specified, duplicate values are removed before aggregation. The listagg function accepts the optional set quantifiers all and distinct: LISTAGG(, …) … 6 To know whether the result is complete or not, users can parse the result 7 or compare the actual length of the result to the calculated length for a result containing all values. The SQL standard does not require a warning to be issued on truncation. If with count is specified and truncation happens, the number of omitted values is put in brackets and appended to the result. ) and will be added as last element if truncation happens. The optional defaults to three periods (. Furthermore, the on overflow truncate clause allows one to specify how the result is terminated: ON OVERFLOW TRUNCATE WITH COUNT The on overflow truncate clause prevents the overflow by only concatenating as many values as the result type can accommodate. In this case, the standard requires an exception with SQLSTATE 22001 to be raised 5- in practice, this requirement is not fulfilled. Listagg accepts the optional on overflow clause to define the behavior if the result exceeds the length limit of the return type: LISTAGG(, ON OVERFLOW …) The return type of listagg is either varchar or clob with an implementation defined length limit. If needed, coalesce can be used to replace null values before aggregation. If no not null value remains, the result of listagg is null. Listagg removes null values before aggregation 2 like most other aggregate functions. 1 Bind parameters are nevertheless well supported in practice. 0 The standard only allows character literals in -i.e. ![]() The must not contain window functions, aggregate functions or subqueries. The minimal syntax is: LISTAGG(, ) WITHIN GROUP(ORDER BY …) Listagg is an ordered set function, which require the within group clause to specify an order. When implementing electronic data interfaces, arrays and document types (JSON, XML) are advantageous as they offer type safety, or at least proper escaping. when aggregating numbers, dates, or strings that are known to not contain the separator. The safe use of listagg for electronic data interfaces is therefore limited to cases in which an unambiguous separator can be selected, e.g. Listagg does not apply any escaping: it is not generally possible to tell whether an occurrence of the separator in the result is an actual separator, or just part of a value. Listagg is typically used to denormalize rows into a string of comma-separated values (CSV) or other comparable formats suitable for human reading. The listagg function transforms values from a group of rows into a list of values that are delimited by a configurable separator.
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