Postgres的SQL递归查询

Question

我使用PostgreSQL 有两个表：图层和图像

LAYERS (layer_id varchar PRIMARY KEY, 
     parent_id varchar REFERENCES LAYERS(layer_id)) 

IMAGES (image_id varchar UNIQUE, REFERENCES LAYERS(layer_id)) 

in LAYERS table: 1->2->3->4->5->6 (1 is the parent of 2) 
in IMAGES table: 1,2,5,6 

现在我想知道，这是图像图像5的最近的父的形象？
答案是图片2

如何编写这样的SQL查询？
非常感谢！

Answer 1

WITH RECURSIVE parents(id) AS(
    SELECT parent FROM layers WHERE layerid = '5' 
    UNION 
    SELECT layers.parent FROM parents, layers WHERE layers.layerid = parents.id 
     AND parents.id NOT IN (SELECT imageid FROM images) 
) 
SELECT imageid FROM parents, images WHERE images.imageid = parents.id;