这应该起作用：

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

你可以试试下面的逻辑-

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

请使用下面的查询，

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';

您也可以使用下面的查询，

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;

你的问题其实很好。几乎。你有一个不必要的 JOIN 在外部查询中：

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';

尽管你要求 left join ，此版本可能具有更好的性能（在 transact_t(merhant_id, transaction_dt) 因为它避免了外部聚集。