问题
I have a Oracle 11g XE database and I have a query the result set:
ID Category
1 Cat1
1 Cat2
2 Cat3
2 Cat4
I want to get distinct id's with all related categories in same row as comma separated like this
ID Categories
1 Cat1,Cat2
2 Cat3,Cat4
I was using Postgres before and array_agg helped me there. How can I get same result in Oracle 11g XE?
回答1:
Unless you are using it in a stored procedure to save the output as an array(or collection), a query with LISTAGG should be sufficient and gives the same output.
select ID , LISTAGG(Category,',') WITHIN GROUP ( ORDER BY ID ) Categories
FROM yourtable GROUP BY ID;
In oracle, we do not have have a straightforward conversion function like array_agg. However, you can create a user-defined collection type, then use CAST and COLLECT functions to convert it to a NESTED TABLE to get the same desired output.
Firstly, create a collection TYPE.
create or replace TYPE categorytype as TABLE OF VARCHAR2(20);
Now, running this query is equivalent to using string_agg or LISTAGG, although categories is an array or collection, rather than a string.
select id, CAST ( COLLECT(Category) as categorytype ) categories
FROM yourtable group by id;
| ID | CATEGORIES |
|----|------------|
| 1 | Cat1,Cat2 |
| 2 | Cat3,Cat4 |
DEMO
回答2:
Oracle has a function listagg() which is pretty close to Postgres's string_agg(). Both seem to do what you want to do.
However, array_agg() creates an array using a SQL expression. I don't think Oracle supports arrays as native types in SQL statements (although arrays are part of PL/SQL). Hence there is no equivalent.
来源：https://stackoverflow.com/questions/47856440/equivalent-of-postgresqls-array-agg-in-oracle-xe-11-2