NULL值型別

count(col_name) 如果col_name的值是NULL，那麼COUNT是不會把它算進去的，所以想統計所有日誌數要使用COUNT(1)

而想對非空列進行相關操作，需要使用col_name IS NOT NULL. 而不是LENGTH(col_name>1), 因為LENGTH(NULL)是沒有結果的

---------------------------------------------------------20170608更新---------------------------------------------------------

除了上述問題，null值在邏輯統計方面也帶來一些麻煩。樓主在使用時，還遇到了如下問題。

問題描述：有條件A、B、C，想統計全部滿足及任意不滿足其中一種情況的下的資料量。

原始程式碼如下：

SELECT
   COUNT(1) AS all_user
,  SUM(CASE WHEN A AND B AND C THEN 1 ELSE 0 END) AS ok_user
,  SUM(CASE WHEN A AND B AND C THEN 0 ELSE 1 END) AS case_user
,  SUM(CASE WHEN !A THEN 1 ELSE 0 END) AS not_A
,  SUM(CASE WHEN !B THEN 1 ELSE 0 END) AS not_B
,  SUM(CASE WHEN !C THEN 1 ELSE 0 END) AS not_C
FROM tb
 

SELECT
   COUNT(1) AS all_user
,  SUM(CASE WHEN A AND B AND C THEN 1 ELSE 0 END) AS ok_user
,  SUM(CASE WHEN A AND B AND C THEN 0 ELSE 1 END) AS case_user
,  SUM(CASE WHEN !A OR A IS NULL THEN 1 ELSE 0 END) AS not_A
,  SUM(CASE WHEN !B OR B IS NULL THEN 1 ELSE 0 END) AS not_B
,  SUM(CASE WHEN !C OR C IS NULL THEN 1 ELSE 0 END) AS not_C
FROM tb
 

上述查詢只能是一個全域性統計，並不能瞭解每一個user的情況。也就是說，如果使用者存在多條記錄，有的記錄是滿足A AND B AND C的，有的是不滿足的，那怎麼統計每種條件下每個使用者的記錄滿足情況呢？～～～～～～～使用MIN和MAX～～～～～～～

SELECT
  COUNT(1) AS all_user
, SUM(ok) AS all_ok
, SUM(not_A) AS all_not_A
, SUM(not_B) AS all_not_B
, SUM(not_C) AS all_not_C
FROM(
    SELECT
      userid
    , MIN(CASE WHEN A AND B AND C THEN 1 ELSE 0 END) AS ok
    , MAX(CASE WHEN !A OR A IS NULL THEN 1 ELSE 0 END) AS not_A
    , MAX(CASE WHEN !B OR B IS NULL THEN 1 ELSE 0 END) AS not_B
    , MAX(CASE WHEN !C OR C IS NULL THEN 1 ELSE 0 END) AS not_C
    FROM tb
    GROUP BY userid
) a

NaN值型別

select col_1, col_2, col_with_nan
from my_table
where some_conditions
  and cast(col_with_nan as String) <> 'NaN';