将内连接的结果限制为1

您可以通过row_number fucntion选择第一行或第一行

    
      WITH x
AS (
    SELECT o.id AS oId
        ,op.id AS opId
        ,op.orderid
        ,op.descr
        ,row_number() OVER (
            PARTITION BY orderid ORDER BY orderid
            ) rows
    FROM ord o
    INNER JOIN orderpos op
        ON op.orderid = o.id
    )
SELECT otd
    ,opid
    ,orderid
    ,DESC
FROM x
WHERE row = 1
</code>
  

那是什么

CROSS APPLY

是用来：

    
      SELECT o.id as oId, op.id as opId, op.orderid, op.descr
FROM ord AS o
CROSS APPLY (   SELECT TOP (1) *
                FROM orderpos AS op
                WHERE op.orderid = o.id
                ORDER BY op.id) AS op;
</code>
  

    
      SELECT 
FROM A, LATERAL( SELECT  FROM B WHERE A.x = B.x ) LDT;
</code>
  

    
      SELECT 
FROM ord, LATERAL( SELECT TOP 1  FROM orderpos WHERE orderpos.orderid = ord.id) LDT;
</code>