用于FLOAT的SQL条件转换VARCHAR

Question

当我运行以下查询时,我收到错误"无法将VARCHAR转换为FLOAT".问题是我的团队中的其他人设计了DB一段时间,并将毫秒作为varchar和sSometime值,因为他将"NA"改为"NA".

现在,当我将毫秒的值转换为AVE()时,"NA"无法生成.有没有办法为"NA"定义默认值,如:

IF a.Milliseconds == "NA" 
     0
ELSE
     CAST(a.Milliseconds AS FLOAT)



SELECT 
  b.Date, 
  AVG(CAST(a.Milliseconds AS FLOAT)) AS Milliseconds,
FROM Fields a
INNER JOIN Cycles b
ON a.CyclesId = b.Id
GROUP BY b.Date

Sample Data

CyclesId | Milliseconds
1        | 24.1557
2        | 23.4886
3        | NA

Answer 1

使用CASE围绕您的价值的陈述

SELECT 
  b.Date, 
  AVG(CAST(CASE WHEN a.Milliseconds = 'NA' THEN '0' ELSE a.Milliseconds END AS FLOAT)) AS Milliseconds,
FROM Fields a
INNER JOIN Cycles b
    ON a.CyclesId = b.Id
GROUP BY b.Date

如果您希望将NA所有值排除在一起,那么您将执行以下操作:

SELECT 
  b.Date, 
  AVG(CAST(a.Milliseconds AS FLOAT)) AS Milliseconds,
FROM Fields a
INNER JOIN Cycles b
    ON a.CyclesId = b.Id
WHERE a.Milliseconds != 'NA'
GROUP BY b.Date

看看SQL Fiddle with Demo