相关疑难解决方法(0)

想要查找某个json列包含空对象的所有行,{}.这可以使用JSON数组,或者如果我在对象中查找特定键.但我只是想知道对象是否为空.似乎找不到会这样做的运营商.

 dev=# \d test
     Table "public.test"
  Column | Type | Modifiers
 --------+------+-----------
  foo    | json |

 dev=# select * from test;
    foo
 ---------
  {"a":1}
  {"b":1}
  {}
 (3 rows)

 dev=# select * from test where foo != '{}';
 ERROR:  operator does not exist: json <> unknown
 LINE 1: select * from test where foo != '{}';
                                      ^
 HINT:  No operator matches the given name and argument type(s). You might need to add explicit type casts.
 dev=# select * from …

我有3张桌子:

users(id, account_balance)
grocery(user_id, date, amount_paid)
fishmarket(user_id, date, amount_paid)

对于具有不同日期和金额的相同user_id,两个fishmarket和grocery表可能有多次出现,或者对于任何给定用户都没有任何内容.当我尝试以下查询时:

SELECT
     t1."id" AS "User ID",
     t1.account_balance AS "Account Balance",
     count(t2.user_id) AS "# of grocery visits",
     count(t3.user_id) AS "# of fishmarket visits"
FROM users t1
LEFT OUTER JOIN grocery t2 ON (t2.user_id=t1."id") 
LEFT OUTER JOIN fishmarket t3 ON (t3.user_id=t1."id") 
GROUP BY t1.account_balance,t1.id
ORDER BY t1.id

它会产生不正确的结果:"1", "12", "12".
但是,当我尝试LEFT JOIN只有一个表时,它会产生正确的结果,无论是访问grocery还是fishmarket访问"1", "3", "4".

我在这做错了什么？
我正在使用PostgreSQL 9.1.

我有下表MyTable:

 id ? value_two ? value_three ? value_four 
???????????????????????????????????????????
  1 ? a         ? A           ? AA
  2 ? a         ? A2          ? AA2
  3 ? b         ? A3          ? AA3
  4 ? a         ? A4          ? AA4
  5 ? b         ? A5          ? AA5

我想查询{ value_three, value_four }按组分组的对象数组value_two.value_two应该在结果中独立存在.结果应如下所示:

 value_two ?                                                                                    value_four                                                                                 
???????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????
 a         ? [{"value_three":"A","value_four":"AA"}, {"value_three":"A2","value_four":"AA2"}, {"value_three":"A4","value_four":"AA4"}]
 b         ? [{"value_three":"A3","value_four":"AA3"}, {"value_three":"A5","value_four":"AA5"}]

它是否使用json_agg()或无关紧要array_agg().

然而,我能做的最好的事情是:

with MyCTE as ( select …