​​    full join 空值填充    ​​        

full join 空值填充

SELECT   coalesce( NULL , 0) as tianchong FROM DUAL 

SELECT   coalesce( '' , '0') as tianchong FROM DUAL      

SELECT A.M, coalesce(B.N, 'X') AS N  FROM (SELECT 0 AS M          FROM DUAL        UNION
        SELECT 1 AS M          FROM DUAL        UNION
        SELECT 2 AS M          FROM DUAL        UNION
        SELECT 3 AS M FROM DUAL) A  FULL JOIN (SELECT 0 AS M, '0' AS N FROM DUAL) B    ON A.M = B.M