MySQL数据库查询：left join 或right join 加上where条件后连接失效，等同于inner join

有一个年都月份临时表，一个 每月金额结果表，通过left join所希望的最终结果为：每个月金额多少，如果这个没有金额，则为0。

月份临时表：

SELECT DISTINCT bgbh,nd,yf FROM  SJQX.t_bg_yflsb WHERE BGBH = 'BG202004260001'

 金额结果表：

SELECT * FROM SJQX.T_BG_00003 T WHERE T.BGBH = 'BG202004260004';

通过LEFT JOIN 语句得到的结果为：

SELECT 
			CASE WHEN T.ynse IS NULL THEN 0 ELSE T.ynse END AS ynse,
			CASE WHEN T.snse IS NULL THEN 0 ELSE T.snse END AS snse,
			T.yf as sj,T.nd,
			CASE WHEN T.sbfs IS NULL THEN '2' ELSE T.sbfs END AS sbfs
			
		FROM (
			SELECT  T1.ynse,T1.snse,T2.yf,T2.nd,T1.sbfs
			FROM 
				(SELECT DISTINCT bgbh,nd,yf FROM  SJQX.t_bg_yflsb WHERE BGBH = 'BG202004260001') t2
				LEFT   JOIN 
				SJQX.T_BG_00003  T1 ON    T1.sm = T2.yf and T1.nd =t2.nd
				WHERE  T1.BGBH = 'BG202004260004'  ) T ORDER BY nd,yf;

没有得到如果在金额表里匹配到得到金额数值，如果没有匹配到则默认为空展示0的效果。

原因：where是先连接然后再筛选 。

解决方法 ：将where 换成and 即：

SELECT 
			CASE WHEN T.ynse IS NULL THEN 0 ELSE T.ynse END AS ynse,
			CASE WHEN T.snse IS NULL THEN 0 ELSE T.snse END AS snse,
			T.yf as sj,T.nd,
			CASE WHEN T.sbfs IS NULL THEN '2' ELSE T.sbfs END AS sbfs
			
		FROM (
			SELECT  T1.ynse,T1.snse,T2.yf,T2.nd,T1.sbfs
			FROM 
				(SELECT DISTINCT bgbh,nd,yf FROM  SJQX.t_bg_yflsb WHERE BGBH = 'BG202004260004') t2
				LEFT   JOIN 
				SJQX.T_BG_00003  T1 ON    T1.sm = T2.yf and T1.nd =t2.nd
				and (T1.BGBH = 'BG202004260004' ) ) T ORDER BY nd,yf;