Mysql 从查询中筛选空值

在下面的查询中，我只想看到风险列中没有空值的行

SELECT table1.id, table1.name,
   CASE 
     WHEN table1.event = 'r' AND table1.name = 'jones' THEN 'very high'
     WHEN table1.event = 't' AND table1.name = 'smith' THEN 'very low'
     ELSE (SELECT table2.risk FROM table2 WHERE table2.value <= table1.value
           ORDER BY table2.value DESC LIMIT 1)
   END AS risk
FROM table1
ORDER BY FIELD( table1.event, 'r', 'f', 't' ), table1.value DESC

我想去掉空的

SELECT id, name, risk FROM (
  SELECT table1.id, table1.name,
   CASE 
     WHEN table1.event = 'r' AND table1.name = 'jones' THEN 'very high'
     WHEN table1.event = 't' AND table1.name = 'smith' THEN 'very low'
     ELSE (SELECT table2.risk FROM table2 WHERE table2.value <= table1.value
           ORDER BY table2.value DESC LIMIT 1)
   END AS risk
  FROM table1
  ORDER BY FIELD( table1.event, 'r', 'f', 't' ), table1.value DESC
) T
WHERE risk IS NOT NULL

更新：我刚刚意识到这是MySQL，而不是一般的SQL问题，所以我不能100%确定MySQL支持这个子查询语法。因此，MySQL方法：

  SELECT table1.id, table1.name,
   CASE 
     WHEN table1.event = 'r' AND table1.name = 'jones' THEN 'very high'
     WHEN table1.event = 't' AND table1.name = 'smith' THEN 'very low'
     ELSE (SELECT table2.risk FROM table2 WHERE table2.value <= table1.value
           AND table2.risk IS NOT NULL
           ORDER BY table2.value DESC LIMIT 1)
   END AS risk
  FROM table1
  ORDER BY FIELD( table1.event, 'r', 'f', 't' ), table1.value DESC

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因此，筛选是在前面完成的，这可能会使用索引，尽管一个好的优化器会将第一个查询转移到第二个查询。

您应该能够使用HAVING子句筛选出这些行：

选择table1.id、table1.name、， 案例 当table1.event='r'和table1.name='jones'时，则为'very high' 当table1.event='t'和table1.name='smith'时，则为'very low' 其他的 选择表2.2风险

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在表2中，我检查了查询，但没有得到正确的结果，而是得到了可重复的行。我用原始查询的结果更新问题。

  SELECT table1.id, table1.name,
   CASE 
     WHEN table1.event = 'r' AND table1.name = 'jones' THEN 'very high'
     WHEN table1.event = 't' AND table1.name = 'smith' THEN 'very low'
     ELSE (SELECT table2.risk FROM table2 WHERE table2.value <= table1.value
           AND table2.risk IS NOT NULL
           ORDER BY table2.value DESC LIMIT 1)
   END AS risk
  FROM table1
  ORDER BY FIELD( table1.event, 'r', 'f', 't' ), table1.value DESC

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