docs: sql function string_agg

Author: gwanhyeon

.DS_Store

@@ -0,0 +1,22 @@
+
+# String_AGG 함수
+
+여러행의 컬럼을 하나로 합치는 함수
+
+`STRING_AGG("합칠컬럼명", "구분자") WITHIN GROUP(ORDER BY "컬럼명")`
+
+
+### 기본 사용법
+SELECT age
+     , STRING_AGG(name, ',') name
+  FROM user
+ WHERE job IN ('A', 'B')
+ GROUP BY age
+ 
+
+### 컬럼 값을 정렬하여 합치는 방법
+SELECT age
+     , STRING_AGG(ename, ',') WITHIN GROUP(ORDER BY name) name
+  FROM user
+ WHERE job IN ('A', 'B')
+ GROUP BY age