Sql 如何添加另一列？

错误：用作表达式的子查询返回多行

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子查询只能返回一行。这意味着您将需要一个聚合、一个TOP 1或WHERE子句，以便只返回1行

select array_to_json(array_agg(row_to_json(req)))::text val 
  from (select (select ST_AsText(wkb_geometry) as wkt 
                  from (SELECT locgeog AS wkb_geometry 
                          from mt_anchor ) as foo)
             , 'Anchor' __type__,* 
          from mt_anchor ) as req;