Postgres中的GROUP BY - JSON数据类型不相等？

Question

我在匹配表中有以下数据：

5;{"Id":1,"Teams":[{"Name":"TeamA","Players":[{"Name":"AAA"},{"Name":"BBB"}]},{"Name":"TeamB","Players":[{"Name":"CCC"},{"Name":"DDD"}]}],"TeamRank":[1,2]}
6;{"Id":2,"Teams":[{"Name":"TeamA","Players":[{"Name":"CCC"},{"Name":"BBB"}]},{"Name":"TeamB","Players":[{"Name":"AAA"},{"Name":"DDD"}]}],"TeamRank":[1,2]}

我想按名称选择表中每个最后一个不同的团队。即我想要一个返回的查询：

6;{"Name":"TeamA","Players":[{"Name":"CCC"},{"Name":"BBB"}
6;{"Name":"TeamB","Players":[{"Name":"AAA"},{"Name":"DDD"}

因此，上次该团队出现在表格中的每个团队。
我一直在使用以下（来自here）：

WITH t AS (SELECT id, json_array_elements(match->'Teams') AS team FROM matches)
SELECT MAX(id) AS max_id, team FROM t GROUP BY team->'Name';

但这会返回：

ERROR: could not identify an equality operator for type json
SQL state: 42883
Character: 1680

我了解 Postgres doesn't have equality for JSON。我只需要队名（一个字符串）的相等性，该队的球员不需要比较。

谁能建议另一种方法来做到这一点？
供参考：

SELECT id, json_array_elements(match->'Teams') AS team FROM matches

返回：

5;"{"Name":"TeamA","Players":[{"Name":"AAA"},{"Name":"BBB"}]}"
5;"{"Name":"TeamB","Players":[{"Name":"CCC"},{"Name":"DDD"}]}"
6;"{"Name":"TeamA","Players":[{"Name":"CCC"},{"Name":"BBB"}]}"
6;"{"Name":"TeamB","Players":[{"Name":"AAA"},{"Name":"DDD"}]}"

编辑：我投射到text 并跟随this question，我使用DISTINCT ON 而不是GROUP BY。这是我的完整查询：

WITH t AS (SELECT id, json_array_elements(match->'Teams') AS team
           FROM matches ORDER BY id DESC)
SELECT DISTINCT ON (team->>'Name') id, team FROM t;

返回上面我想要的。谁有更好的解决方案？

Answer 1

通过LATERAL 加入更短、更快、更优雅：

SELECT DISTINCT ON (t.team->>'Name') t.team
FROM   matches m, json_array_elements(m.match->'Teams') t(team);
ORDER  BY t.team->>'Name', m.id DESC;  -- to get the "last"

如果您只想要不同的团队，ORDER BY 可以去。相关：

• Query for element of array in JSON column
• Query for array elements inside JSON type

JSON 和相等性

Postgres 中没有json 数据类型的相等运算符，但jsonb 有一个相等运算符（Postgres 9.4+）：

• How to query a json column for empty objects?