Google bigquery 没有意义。你的场景中有1个条件连接，更新后的版本实际上满足了我上面的理论。join说它肯定需要扫描t1和t2。case语句中包含t2的原始连接没有定义t2的扫描量。@rtenha但是包含两个连接的那一个呢？如果你的理论是正确的，那不是也行吗？这是同一个问题

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没有意义。你的场景中有1个条件连接，更新后的版本实际上满足了我上面的理论。join说它肯定需要扫描t1和t2。case语句中包含t2的原始连接没有定义t2的扫描量。@rtenha但是包含两个连接的那一个呢？如果你的理论是正确的，那不是也行吗？这是同一个问题。t3在case语句中，使其具有条件。它无法估计需要扫描多少t3。

#standardSQL
SELECT *,
  ARRAY(
    SELECT AS STRUCT *  
    FROM t2 b
    WHERE b.id IN (a.id, a.other_id) 
    ORDER BY (
      CASE
        WHEN dairy IN ('milk', 'yogurt') THEN 1
        ELSE 2
      END    
    )
    LIMIT 1
  )[SAFE_OFFSET(0)] AS t2  
FROM t1 a  

Row dairy   id  other_id    t2.color    t2.id    
1   milk    1   2           blue        1    
2   yogurt  3   4           red         4    
3   cheese  5   6           

WITH t1 AS (
  SELECT "milk" AS dairy, 1 AS id, 2 AS other_id UNION ALL
  SELECT "yogurt", 3, 4 UNION ALL
  SELECT "cheese", 5, 6
),
t2 AS (
  SELECT "blue" AS color, 1 AS id UNION ALL
  SELECT "red", 4
),
t3 AS (
  SELECT "sunny" AS weather, 1 as id, 10 as other_id UNION ALL
  SELECT "cloudy", 11, 4
),
join_t1_t2 as (
  select
    t1.*,
    case 
      when t1.dairy = 'milk' then milk.color
      when t1.dairy = 'yogurt' then yogurt.color
      else null
    end as t2_color,
    case 
      when t1.dairy = 'milk' then milk.id
      when t1.dairy = 'yogurt' then yogurt.id
      else null
    end as t2_id
  from t1
  left join t2 milk on t1.id = milk.id
  left join t2 yogurt on t1.other_id = yogurt.id
),
join_t1_t2_t3 as (
  select
    join_t1_t2.*,
    case 
      when t2_color = 'blue' then blue.id
      when t2_color = 'red' then red.id
      else null
    end as t3_id,
    case 
      when t2_color = 'blue' then blue.other_id
      when t2_color = 'red' then red.other_id
      else null
    end as t3_other_id,
    case 
      when t2_color = 'blue' then blue.weather
      when t2_color = 'red' then red.weather
      else null
    end as t3_weather,
  from join_t1_t2
  left join t3 blue on t2_id = blue.id
  left join t3 red on t2_id = red.other_id
)
select * from join_t1_t2_t3