SQL 更改子查询以加入答案

【问题标题】：SQL change subquery to joinSQL 更改子查询以加入
【发布时间】：2020-05-28 09:05:10
【问题描述】：

我有以下查询，它有一个子查询。我需要将子查询转换为带有日期分隔的“where”子句的连接，这将在 Excel 电子表格中进行参数化。

    select me.id, me.merchant_num, me.merchant_nm,
    (select count(1) from transaction_t where merchant_id = me.id and transaction_dt BETWEEN '2020-04-01' and '2020-04-30') as num_transactions
    FROM merchant_t me
    left outer join transaction_t tt on tt.merchant_id = me.id
    where me.status = 'T'

子查询获取给定日期之间所有商家交易的计数。我已经尝试了所有我能想到的方法，但要么我得到的行太多，要么有其他问题。

涉及两个表：

merchant_t
----------
id     merchant_num    merchant_nm    status


transaction_t
--------------
id     merchant_id     transaction_dt

【问题讨论】：

标签： sql sql-server

【解决方案1】：

这应该可行：

    SELECT me.id, me.merchant_num, me.merchant_nm, count(1) as num_transactions
    FROM merchant_t me
    LEFT OUTER JOIN transaction_t
         ON t.merchant_id = me.id 
         AND t.transaction_dt BETWEEN '2020-04-01' and '2020-04-30'
    WHERE me.status = 'T'
    GROUP BY me.id, me.merchant_num, me.merchant_nm

【讨论】：

【解决方案2】：

你可以试试下面这个逻辑-

select me.id, 
me.merchant_num, 
me.merchant_nm,
count(tt.merchant_id) as num_transactions
FROM merchant_t me
left outer join transaction_t tt 
    on tt.merchant_id = me.id
    and tt.transaction_dt BETWEEN '2020-04-01' and '2020-04-30'
    and me.status = 'T'
group by me.id, 
me.merchant_num, 
me.merchant_nm

【讨论】：

【解决方案3】：

请使用以下查询，

select me.id, me.merchant_num, me.merchant_nm, qry.num_transactions from merchant_t me
left outer join (select merchant_id, count(1) as  num_transactions from transaction_t where transaction_dt BETWEEN '2020-04-01' and '2020-04-30' group by merchant_id) qry
on (me.id = qry.merchant_id)
where me.status = 'T';

您也可以使用以下查询，

select me.id, me.merchant_num, me.merchant_nm, qry.num_transactions, count(1) from merchant_t me
 left outer join transaction_t qry
on (me.id = qry.merchant_id)
where me.status = 'T'
group by me.id, me.merchant_num, me.merchant_nm, qry.num_transactions;

【讨论】：

【解决方案4】：

您的查询实际上很好。 . .几乎。您在外部查询中有一个不必要的JOIN：

select me.id, me.merchant_num, me.merchant_nm,
      (select count(1)
       from transaction_t
       where t.merchant_id = me.id and
             t.transaction_dt between '2020-04-01' and '2020-04-30'
     ) as num_transactions
from merchant_t me
where me.status = 'T';

虽然您要求left join，但此版本可能具有更好的性能（在transact_t(merhant_id, transaction_dt) 上有一个索引，因为它避免了外部聚合。

【讨论】：