Question

对于MySQL而言，我是自学成才，我不确定这样做的正确方法是什么：

$type1_sql          ="SELECT * FROM type1 WHERE type1_id=$type1_id";    
$type1_res          =mysqli_query($handle_db, $type1_sql);
$type1_row          =mysqli_fetch_array($type1_res);
$type1_name         =$type1_row['name'];

$type2_sql          ="SELECT * FROM type2 WHERE type2_id=$type2_id";    
$type2_res          =mysqli_query($handle_db, $type2_sql);
$type2_row          =mysqli_fetch_array($type2_res);
$type2_name         =$type2_row['name'];

$type3_sql          ="SELECT * FROM type3 WHERE type3_id=$type3_id";    
$type3_res          =mysqli_query($handle_db, $type3_sql);
$type3_row          =mysqli_fetch_array($type3_res);
$type3_name         =$type3_row['name'];

做我正在做的事情会更好吗？单独查询所有内容，或者有没有办法将所有这些内容压缩成一个查询？

谢谢，杰森

Answer 1

如果您只需要每个表的name列，则可以使用SELECT UNION查询：

(SELECT name FROM type1 WHERE type1_id=$type1_id)
UNION
(SELECT name FROM type2 WHERE type2_id=$type2_id)
UNION
(SELECT name FROM type3 WHERE type3_id=$type3_id);

Answer 2

重要提示：考虑到虽然@Alp的解决方案可能正是您所需要的：

(SELECT name FROM type1 WHERE type1_id=$type1_id)
UNION
(SELECT name FROM type2 WHERE type2_id=$type2_id);
UNION
(SELECT name FROM type3 WHERE type3_id=$type3_id);

如果$type1_id等于$type2_id，它将删除可能找到的所有重复项。如果要完全选择所有结果（包括重复行），则应运行此操作（删除不必要的括号）：

SELECT name FROM type1 WHERE type1_id=$type1_id
UNION ALL
SELECT name FROM type2 WHERE type2_id=$type2_id
UNION ALL
SELECT name FROM type3 WHERE type3_id=$type3_id;

MySQL - 冷凝查询

2 个答案: