使用左连接选择一对多关系中的第一条记录

I'm trying to join two tables using a left-join. And the result set has to include only the first record from the "right" joined table.

我正在尝试使用左连接来连接两个表。结果集必须只包含“右”连接表中的第一条记录。

Lets say I have two tables A and B as below;

可以说我有两张表A和B如下;

Table "A"

表“A”

code | emp_no

101  | 12222
102  | 23333
103  | 34444
104  | 45555
105  | 56666

Table "B"

表“B”

code | city       | county
101  | Glen Oaks  | Queens
101  | Astoria    | Queens
101  | Flushing   | Queens
102  | Ridgewood  | *lyn
103  | Bayside    | New York

Expected Output:

预期产出：

code | emp_no | city      | county
101  | 12222  | Glen Oaks | Queens
102  | 23333  | Ridgewood | *lyn
103  | 34444  | Bayside   | New York
104  | 45555  | NULL      | NULL
105  | 56666  | NULL      | NULL

If you notice my result has only the one matched record from table "B"(doesn't matter what record is matched) after left join (and it is a one to many mapping)

如果你注意到我的结果在左连接之后只有表“B”中的一个匹配记录（无论匹配什么记录）（并且它是一对多的映射）

I need to pick the first matched record from table B and ignore all other rows.

我需要从表B中选择第一个匹配的记录并忽略所有其他行。

Please help!

请帮忙！

Thanks

谢谢

7 个解决方案

#1

After playing around a bit, this turns out to be trickier than I'd expected! Assuming that table_b has some single column that is unique (say, a single-field primary key), it looks like you can do this:

玩了一下之后，事实证明这比我想象的要复杂！假设table_b有一些唯一的列（例如，单字段主键），看起来你可以这样做：

SELECT table_a.code,
       table_a.emp_no,
       table_b.city,
       table_b.county
  FROM table_a
  LEFT
  JOIN table_b
    ON table_b.code = table_a.code
   AND table_b.field_that_is_unique =
        ( SELECT TOP 1
                 field_that_is_unique
            FROM table_b
           WHERE table_b.code = table_a.code
       )
;

#2

Another option: OUTER APPLY

If supported by the database, OUTER APPLY is an efficient and terse option.

如果数据库支持，OUTER APPLY是一个高效且简洁的选项。

SELECT *
FROM 
    Table_A a
OUTER APPLY
    (SELECT TOP 1 * 
    FROM Table_B b_1
    WHERE b_1.code = a.code
    ) b
;

This results in a left join to the indeterminate first matched record. My tests show it to be quicker than any other posted solution (on MS SQL Server 2012).

这导致左连接到不确定的第一匹配记录。我的测试显示它比任何其他发布的解决方案更快（在MS SQL Server 2012上）。

#3

If you are on SQL Server 2005 or later version, you could use ranking to achieve what you want. In particular, ROW_NUMBER() seems to suit your needs nicely:

如果您使用的是SQL Server 2005或更高版本，则可以使用排名来实现所需。特别是，ROW_NUMBER（）似乎很适合您的需求：

WITH B_ranked AS (
  SELECT
    *,
    rnk = ROW_NUMBER() OVER (PARTITION BY code ORDER BY city)
  FROM B
)
SELECT
  A.code,
  A.emp_no,
  B.city,
  B.county
FROM A
  LEFT JOIN B_ranked AS B ON A.code = B.code AND b.rnk = 1

OR

要么

WITH B_unique_code AS (
  select * from(
     SELECT
      *,
      rnk = ROW_NUMBER() OVER (PARTITION BY code ORDER BY city)
      FROM B
     ) AS s
  where rnk = 1
)
SELECT
  A.code,
  A.emp_no,
  B.city,
  B.county
FROM A
  LEFT JOIN B_unique_code AS B ON A.code = B.code

#4

The highest voted answer does not seem correct to me, and seems overcomplicated. Just group by the code field on table B in your subquery and select the maximum Id per grouping.

最高投票的答案对我来说似乎不正确，而且似乎过于复杂。只需按子查询中表B上的代码字段进行分组，然后选择每个分组的最大Id。

SELECT 
    table_a.code,
    table_a.emp_no,
    table_b.city,
    table_b.county
FROM 
    table_a
    LEFT JOIN 
        table_b
        ON table_b.code = table_a.code
        AND table_b.field_that_is_unique IN
            (SELECT MAX(field_that_is_unique)
             FROM table_b
             GROUP BY table_b.code)

#5

I modified the answer from ruakh and this seem to work perfectly with mysql.

我修改了ruakh的答案，这似乎与mysql完美配合。

SELECT 
   table_a.code,
   table_a.emp_no,
   table_b.city,
   table_b.county
FROM table_a a
LEFT JOIN table_b b
ON b.code = a.code 
AND b.id = (  SELECT id FROM table_b 
              WHERE table_b.code = table_a.code 
              LIMIT 1
           )
;

#6

In Oracle you can do:

在Oracle中，您可以：

WITH first_b AS (SELECT code, min(rowid) AS rid FROM b GROUP BY code)) 
SELECT a.code, a.emp_no, b.city, b.county
FROM a 
INNER JOIN first_b 
 ON first_b.code = a.code
INNER JOIN b
 ON b.rowid = first_b.rid

#7

this is how:

这是如何：

 Select * From TableA a
     Left Join TableB b
         On b.Code = a.Code 
             And [Here put criteria predicate that 'defines' what the first record is]

Hey, if the city and county are unique, then use them

嘿，如果这个城市和县是独一无二的，那就用它们吧

   Select * From TableA a
     Left Join TableB b
         On b.Code = a.Code 
             And b.City + b.county =
                  (Select Min(city + county)
                   From TableB 
                   Where Code = b.Code)

But the point is you have to put some expression in there to tell the query processor what it means to be first.

但重点是你必须在那里放一些表达式来告诉查询处理器它首先意味着什么。