SQL Query to concatenate column values from multip

Question

Would it be possible to construct SQL to concatenate column values from multiple rows?

The following is an example:

Table A

PID
A
B
C

Table B

PID   SEQ    Desc

A     1      Have
A     2      a nice
A     3      day.
B     1      Nice Work.
C     1      Yes
C     2      we can 
C     3      do 
C     4      this work!

Output of the SQL should be -

PID   Desc
A     Have a nice day.
B     Nice Work.
C     Yes we can do this work!

So basically the Desc column for out put table is a concatenation of the SEQ values from Table B?

Any help with the SQL?

Answer 1

As most of the answers suggest, LISTAGG is the obvious option. However, one annoying aspect with LISTAGG is that if the total length of concatenated string exceeds 4000 characters( limit for VARCHAR2 in SQL ), the below error is thrown, which is difficult to manage in Oracle versions upto 12.1

ORA-01489: result of string concatenation is too long

A new feature added in 12cR2 is the ON OVERFLOW clause of LISTAGG. The query including this clause would look like:

SELECT pid, LISTAGG(Desc, ' ' on overflow truncate) WITHIN GROUP (ORDER BY seq) AS desc
FROM B GROUP BY pid;

The above will restrict the output to 4000 characters but will not throw the ORA-01489 error.

These are some of the additional options of ON OVERFLOW clause:

• ON OVERFLOW TRUNCATE 'Contd..' : This will display 'Contd..' at the end of string (Default is ... )
• ON OVERFLOW TRUNCATE '' : This will display the 4000 characters without any terminating string.
• ON OVERFLOW TRUNCATE WITH COUNT : This will display the total number of characters at the end after the terminating characters. Eg:- '...(5512)'
• ON OVERFLOW ERROR : If you expect the LISTAGG to fail with the ORA-01489 error ( Which is default anyway ).

Answer 2

1. LISTAGG delivers the best performance if sorting is a must(00:00:05.85)

   SELECT pid, LISTAGG(Desc, ' ') WITHIN GROUP (ORDER BY seq) AS description FROM B GROUP BY pid;

2. COLLECT delivers the best performance if sorting is not needed(00:00:02.90):

   SELECT pid, TO_STRING(CAST(COLLECT(Desc) AS varchar2_ntt)) AS Vals FROM B GROUP BY pid;

3. COLLECT with ordering is bit slower(00:00:07.08):

   SELECT pid, TO_STRING(CAST(COLLECT(Desc ORDER BY Desc) AS varchar2_ntt)) AS Vals FROM B GROUP BY pid;

All other techniques were slower.

Answer 3

There are a few ways depending on what version you have - see the oracle documentation on string aggregation techniques. A very common one is to use LISTAGG:

SELECT pid, LISTAGG(Desc, ' ') WITHIN GROUP (ORDER BY seq) AS description
FROM B GROUP BY pid;

Then join to A to pick out the pids you want.

Note: Out of the box, LISTAGG only works correctly with VARCHAR2 columns.

Answer 4

Before you run a select query, run this:

SET SERVEROUT ON SIZE 6000

SELECT XMLAGG(XMLELEMENT(E,SUPLR_SUPLR_ID||',')).EXTRACT('//text()') "SUPPLIER" 
FROM SUPPLIERS;

Answer 5

There's also an XMLAGG function, which works on versions prior to 11.2. Because WM_CONCAT is undocumented and unsupported by Oracle, it's recommended not to use it in production system.

With XMLAGG you can do the following:

SELECT XMLAGG(XMLELEMENT(E,ename||',')).EXTRACT('//text()') "Result" 
FROM employee_names

What this does is

• put the values of the ename column (concatenated with a comma) from the employee_names table in an xml element (with tag E)
• extract the text of this
• aggregate the xml (concatenate it)
• call the resulting column "Result"

Answer 6

The LISTAGG analytic function was introduced in Oracle 11g Release 2, making it very easy to aggregate strings. If you are using 11g Release 2 you should use this function for string aggregation. Please refer below url for more information about string concatenation.

http://www.oracle-base.com/articles/misc/StringAggregationTechniques.php

String Concatenation