PostgreSql的聚合函数string_agg和自定义聚合函数group_concat

1.使用自定义聚合函数  group_concat

SELECT * FROM
	(            
	SELECT "FFID",CAST ("BRDNO" AS INTEGER),"count" (*) AS "NUM",
		array_to_string(group_concat("PRSTA"), ',') AS "PRSTA"
		FROM "metaHeader" A,"PNOPPNCK" b
		WHERE
		A .metaid = b.metaid
		AND "FLIGHTDATE" = '20181028'
		GROUP BY "FFID","BRDNO"
		)
ORDER BY "FFID","BRDNO"

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2. 使用自带聚合函数 string_agg查询

说明：string_agg更加耗时。

--1.自定义聚合函数group_concat
 CREATE AGGREGATE group_concat(anyelement)
(
    sfunc = array_append, -- 每行的操作函数，将本行append到数组里 
    stype = anyarray,  -- 聚集后返回数组类型 
    initcond = '{}'    -- 初始化空数组
);