在一对多关系中进行内部连接时只想要一行

Question

我相信它们是我问题的简单答案。

我有 2 个表——一个订单头表（每个订单一行，称为"订单

"）和一个订单明细表（每个订单头 1 行或更多行，称为"Order_Line"）。 我正在使用 Sql Server2005 及其 SSRS 报告工具。 我只想返回一行，但我得到了每个订单的详细信息表中有多少行。 我需要联接到明细表，才能获得要与其他订单头级别项目一起显示的值。

我尝试选择不同的，但得到了相同的结果。 我的查询是：

select o.order_num, o.cust_num, c.cust_desc, o.shipto_num, o.st_name, o.st_address1, o.ent_date, 
o.ord_date, o.req_date, ol.order_line, ol.seq_num, u.user_desc, ol.orig_inv_num, ol.prod_num,
p.prod_desc1, ol.prod_desc1, ol.prod_desc2, ol.grs_price, ol.disc_pct, ol.net_price, ol.ord_qty,
ol.opn_qty, ol.pck_qty, ol.bo_qty, ol.shp_qty, rs.rsn_code_desc, ol.supp_cde, ol.line_ctr,
v.vend_desc, ca.pline_desc, ca.major_grp, ol.major_grp, w.whse_desc, o.wb,s.ship_via_desc,       
sa.slsm_desc, t.ar_term_desc,
o.shp_date, o.sys_num, o.inv_Flag, o.inv_num, o.tot_ord_$,o.inv_date, o.ord_src, r.route_desc,
o.cred_type, o.cred_num, o.total_sale
from [order] o
left outer join order_line ol on o.order_num = ol.order_num
left outer join customer c on o.cust_num = c.cust_no
left outer join ship_via s on o.ship_via_num = s.ship_via_id
left outer join whse_addr w on o.shp_whse = w.whse_num
left outer join salesman sa on o.slsm_num = sa.slsm_num
left outer join terms t on o.ar_term_num = t.ar_term_num
left outer join [route] r on o.route_num = r.route_num
left outer join product p on ol.prod_num = p.prod_num
left outer join prc_ov_rsn rs on ol.prc_ov_rsn_code = rs.rsn_cod
left outer join vend v on ol.prim_vend_num = v.vend_num
left outer join [user_id] u on ol.[user_id] = u.[user_num] 
left outer join category ca on ol.pline_Num = ca.pline_Id
where o.cust_num = '606797'
order by o.cust_num, o.shipto_num, o.order_num

如何让查询只返回一行，而不管每个订单有多少条Order_Line记录？

Answer 1

我将尝试从一般的SQL角度回答这个问题，因为我不知道SQL Server本身。希望这是有道理的，但如果我完全关闭，请告诉我，我会杀死这个答案。

您可以尝试解决此问题的一种方法是创建一个使用 LIMIT 1 的依赖子查询。因此，联接最终将仅与一个订单详细信息行匹配。这样：

select ...all columns you want...
from [order] o
left outer join order_line ol on o.order_num = ol.order_num
left outer join customer c on o.cust_num = c.cust_no
left outer join ship_via s on o.ship_via_num = s.ship_via_id
left outer join whse_addr w on o.shp_whse = w.whse_num
left outer join salesman sa on o.slsm_num = sa.slsm_num
left outer join terms t on o.ar_term_num = t.ar_term_num
left outer join [route] r on o.route_num = r.route_num
left outer join product p on ol.prod_num = p.prod_num
left outer join prc_ov_rsn rs on ol.prc_ov_rsn_code = rs.rsn_cod
left outer join vend v on ol.prim_vend_num = v.vend_num
left outer join [user_id] u on ol.[user_id] = u.[user_num] 
left outer join category ca on ol.pline_Num = ca.pline_Id
where o.cust_num = '606797' AND order_line.order_num =
    (SELECT order_num 
     FROM order_line 
     WHERE order_line.order_num = order.order_num LIMIT 1)
order by o.cust_num, o.shipto_num, o.order_num

结果是，您将分别获取每个订单行以及从order_line表中选择的列，但由于已将其限制为每行 1 order_line，因此结果集中每个订单只应获取一行。

通过适当的索引等，希望这不会导致任何显着的额外加载时间。让我知道这是否有意义。

注意：显然SQL Server没有像Oracle，MySQL，PostgreSQL，SQLite那样的limit子句。等等等等。我找到了看起来像SELECT TOP 1...的东西的引用，但我不知道这是否是等效的。任何阅读本文的人都在寻找类似的答案，显然您必须用该特定 DBMS 中语法正确的任何内容替换LIMIT 1。并让我知道表达行数限制的正确方法。