sql - 如何在sql查询中使用group by？

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

当我运行查询时，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.

如何写这个查询？

Answer 1

错误很明显，要么使用带有 t2.name 的聚合函数，要么将其添加到 GROUP BY，这取决于您要查看的期望结果对于:

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;

这个错误是有道理的，因为它必须知道从 t2.name 中为每组 t1.name 选择哪个值？它应该选择 max、min 等。否则 GROUP BY 它。

此外，删除 DISTINCT，GROUP BY 不需要它。