5.mysql 连接字符串为空时的处理

1. 查出字段为空的判断
   
   • isnull
     isnull(xxxx, ‘default’),可以使用isnull函数进行判断，‘default’ 为替换字符，select * from table where isnull(xxx,'') = ''；
   • length
     length(xxxx) 函数判断查询出的字段的长度进行判断；
   • is not null
     在语句中包含 select * from table where xxx is not null；
   • caue when
     使用case when 语句 (cause when xx is null or xx = '' then '为空后返回该字符串' else xx)
   • case when + count
     加上count函数进行判断(cause when count(xx) = 0 then '为空后返回该字符串' else xx)
2. 在sql查询中有些时候就要使用concat函数进行字符串的拼接，
   例如：

SELECT CONCAT(a.str1,b.str1) FROM(SELECT     (CASE WHEN count(elmtGroupName) = 0 THEN '' ELSE CONCAT(elmtGroupName,':',    (CASE WHEN count(priceImpact) = 0 THEN '' ELSE priceImpact  END),';') END)     as str1,elmtGroupName FROM report_otherinfoWHERE elmtGroupName LIKE '主原料%') AS a,(SELECT     (CASE WHEN count(elmtGroupName) = 0 THEN '' ELSE CONCAT(elmtGroupName,':',    (CASE WHEN count(priceImpact) = 0 THEN '' ELSE priceImpact  END),';') END) as str1,elmtGroupName FROM report_otherinfoWHERE elmtGroupName LIKE '按时%') AS b;