sql - SQL - 选择 - 外键

Question

VISITSEXIT              VISITSENTRY               VISITOR
idvisitsexit            idvisitsentry             idvisitor 
idvisitsentry           idvisitante               visitor 
idvisitor               visitor
visitor
dateexit
timeexit

我需要选择表格VISITSEXIT，当她带上VISITSENTRY 时也会带上idvisitor 和idvisitor（访问者）的描述。

Answer 1

一个简单的直接JOIN会给你你想要的：

SELECT
  t.idvisitor,
  t.visitor,
  t.dateexit,
  t.timeext,
  ... -- the rest of the columns you want to select 
FROM VISITSEXIT       AS t
INNER JOIN VisitEntry AS e ON t.idvisitentry = e.visitentry
INNER JOIN Visitor    AS v ON e.idvisitante  = v.idvisitor;

LEFT JOIN如果您想包含其他表中那些不匹配的行，您可能还需要使用。有关JOIN类型的更多信息，请参阅此帖子：

• SQL 连接的可视化解释