如何在SQL查询中使用分组方式？

Question

SELECT DISTINCT 
           t1.name as t1_name, 
           MAX(t1.unit) as t1_unit, 
           MAX(t1.id_producer_goods) AS hi_id_producer_goods, 
          t2.name as t2_name 
FROM Table1 t1 
    left join Table2 t2 on t1.id_web_site=t2.id_web_site 
WHERE t1.id='23'  
GROUP BY t1.name

运行查询时，出现以下错误：

Column 'Table2.name' is invalid in the select list because it is not contained 
in either an aggregate function or the GROUP BY clause.

如何编写此查询？

Answer 1

错误非常明显，可以将聚合函数与t2.name一起使用，或者将其添加到GROUP BY ，这取决于您要查找的期望结果：

SELECT 
  t1.name as t1_name, 
  t2.name as t2_name,
  MAX(t1.unit) as t1_unit, 
  MAX(t1.id_producer_goods) AS hi_id_producer_goods
FROM Table1 hi 
left join Table2 t2 on t1.id_web_site=t2.id_web_site 
WHERE t1.id='23'  
GROUP BY t1.name, t2.name;

该错误是有道理的，因为它必须知道要为每个t1.name组从t2.name选择哪个值？ 是否应选择max ， min等，否则应使用GROUP BY 。

另外，使用GROUP BY删除不需要的DISTINCT 。

Answer 2

SELECT 
t1.name as t1_name, 
MAX(t1.unit) as t1_unit, 
MAX(t1.id_producer_goods) AS hi_id_producer_goods, 
t2.name as t2_name FROM Table1 hi 
left join Table2 t2 on t1.id_web_site=t2.id_web_site 
WHERE 
t1.id='23'  
GROUP BY t1.name,t2.name

您需要按AGG函数中未使用的所有字段分组。 等MAX

Answer 3

尝试这个

SELECT 
DISTINCT 
t1.name as t1_name, 
MAX(t1.unit) as t1_unit, 
MAX(t1.id_producer_goods) AS hi_id_producer_goods, 
t2.name as t2_name FROM Table1 hi 
left join Table2 t2 on t1.id_web_site=t2.id_web_site 
WHERE 
t1.id='23'  
GROUP BY t1.name,t2.name

您正在使用具有2列的聚合函数，那么您需要对这两列进行分组

Answer 4

我认为您不需要在这里使用DISTINCT关键字。

           SELECT 
           t1.name as t1_name, 
           MAX(t1.unit) as t1_unit, 
           MAX(t1.id_producer_goods) AS hi_id_producer_goods, 
          t2.name as t2_name 
FROM Table1 t1 
    left join Table2 t2 on t1.id_web_site=t2.id_web_site 
WHERE t1.id='23'  
GROUP BY t1.name,t2.name