思路分析：通过为每一队分配一个id，join条件要求t1.num < t2.num实现相同两队只比一次

代码实现：

with t as (
    SELECT team_name,
        case
            team_name
            WHEN '勇士' then 1
            WHEN '湖人' then 2
            WHEN '灰熊' then 3
            else 4
        end num
    FROM team
)

SELECT 
    t1.team_name team_name_1,
    t2.team_name team_name_2
FROM t t1
    JOIN t t2
WHERE t1.num < t2.num