工作中用到一段比較複雜的SQL查詢指令碼，使用了listagg()函式實現了具有多個值的欄位的填充（即，列表聚合，list aggregation（我猜的））。

說簡單點，listagg()函式可以實現多列記錄聚合為一條記錄，從而實現資料的壓縮、緻密化（data densification）。

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這是一個Oracle的列轉行函式：LISTAGG()

先看示例程式碼： 

with temp as(  
select 'China' nation ,'Guangzhou' city from dual union all  
select 'China' nation ,'Shanghai' city from dual union all  
select 'China' nation ,'Beijing' city from dual union all  
select 'USA' nation ,'New York' city from dual union all  
select 'USA' nation ,'Bostom' city from dual union all  
select 'Japan' nation ,'Tokyo' city from dual   
)  
select nation,listagg(city,',') within GROUP (order by city)  as Cities
from temp  
group by nation
 

執行結果：

這是最基礎的用法：

LISTAGG(XXX,XXX) WITHIN GROUP( ORDER BY XXX)，

用法就像聚合函式一樣，通過Group by語句，把每個Group的一個欄位，拼接起來，非常方便。 

同樣是聚合函式，還有一個高階用法：

就是over(partition by XXX)

也就是說，在你不使用Group by語句時候，也可以使用LISTAGG函式：

with temp as(  
select 500 population, 'China' nation ,'Guangzhou' city from dual union all  
select 1500 population, 'China' nation ,'Shanghai' city from dual union all  
select 500 population, 'China' nation ,'Beijing' city from dual union all  
select 1000 population, 'USA' nation ,'New York' city from dual union all  
select 500 population, 'USA' nation ,'Bostom' city from dual union all  
select 500 population, 'Japan' nation ,'Tokyo' city from dual   
)  
select population,  
nation,  
city,  
listagg(city,',') within GROUP (order by city) over (partition by nation) rank  
from temp
 

總結：LISTAGG()把它當作SUM()函式來使用就可以了。

Oracle Database SQL Language Reference上有關listagg()函式的描述如下：

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Purpose
For a specified measure, LISTAGG orders data within each group specified in the ORDER BY clause and then concatenates the values of the measure column.
■ As a single-set aggregate function, LISTAGG operates on all rows and returns a single output row.
■ As a group-set aggregate, the function operates on and returns an output row for each group defined by the GROUP BY clause.
■ As an analytic function, LISTAGG partitions the query result set into groups based on one or more expression in the query_partition_clause.
The arguments to the function are subject to the following rules:
■ The measure_expr can be any expression. Null values in the measure column are ignored.
■ The delimiter_expr designates the string that is to separate the measure values.
This clause is optional and defaults to NULL.
■ The order_by_clause determines the order in which the concatenated values are returned. The function is deterministic only if the ORDER BY column list achieved
unique ordering.
The return data type is RAW if the measure column is RAW; otherwise the return value is VARCHAR2.
Aggregate Examples
The following single-set aggregate example lists all of the employees in Department 30 in the hr.employees table, ordered by hire date and last name:
SELECT LISTAGG(last_name, '; ')
WITHIN GROUP (ORDER BY hire_date, last_name) "Emp_list",
MIN(hire_date) "Earliest"
FROM employees
WHERE department_id = 30;
Emp_list Earliest
------------------------------------------------------------ ---------
Raphaely; Khoo; Tobias; Baida; Himuro; Colmenares 07-DEC-02
The following group-set aggregate example lists, for each department ID in the hr.employees table, the employees in that department in order of their hire date:

SELECT department_id "Dept.",
LISTAGG(last_name, '; ') WITHIN GROUP (ORDER BY hire_date) "Employees"
FROM employees
GROUP BY department_id
ORDER BY department_id;
Dept. Employees
------ ------------------------------------------------------------
10 Whalen
20 Hartstein; Fay
30 Raphaely; Khoo; Tobias; Baida; Himuro; Colmenares
40 Mavris
50 Kaufling; Ladwig; Rajs; Sarchand; Bell; Mallin; Weiss; Davie
s; Marlow; Bull; Everett; Fripp; Chung; Nayer; Dilly; Bissot
; Vollman; Stiles; Atkinson; Taylor; Seo; Fleaur; Matos; Pat
el; Walsh; Feeney; Dellinger; McCain; Vargas; Gates; Rogers;
Mikkilineni; Landry; Cabrio; Jones; Olson; OConnell; Sulliv
an; Mourgos; Gee; Perkins; Grant; Geoni; Philtanker; Markle
60 Austin; Hunold; Pataballa; Lorentz; Ernst
70 Baer
. . .
Analytic Example
The following analytic example shows, for each employee hired earlier than September 1, 2003, the employee's department, hire date, and all other employees in
that department also hired before September 1, 2003:
SELECT department_id "Dept", hire_date "Date", last_name "Name",
LISTAGG(last_name, '; ') WITHIN GROUP (ORDER BY hire_date, last_name)
OVER (PARTITION BY department_id) as "Emp_list"
FROM employees
WHERE hire_date < '01-SEP-2003'
ORDER BY "Dept", "Date", "Name";
Dept Date Name Emp_list
----- --------- --------------- ---------------------------------------------
30 07-DEC-02 Raphaely Raphaely; Khoo
30 18-MAY-03 Khoo Raphaely; Khoo
40 07-JUN-02 Mavris Mavris
50 01-MAY-03 Kaufling Kaufling; Ladwig
50 14-JUL-03 Ladwig Kaufling; Ladwig
70 07-JUN-02 Baer Baer
90 13-JAN-01 De Haan De Haan; King
90 17-JUN-03 King De Haan; King
100 16-AUG-02 Faviet Faviet; Greenberg
100 17-AUG-02 Greenberg Faviet; Greenberg
110 07-JUN-02 Gietz Gietz; Higgins
110 07-JUN-02 Higgins Gietz; Higgins