Question

我想从两个用户名表中找出我的用户名，其中用户名已经停留在table2中。这意味着如果任何表中的用户名匹配，我的查询将收集其用户ID。

table 1:  username --- userid  > john --- 100

table 2:  username --- userid  > colin --- 101

我在这里看了很多文章，我无法理解我该怎么做。

现在想想：我需要约翰的uesrid，

所以我试过了：

$username = "john";

$q = "SELECT userid FROM table1,table2 WHERE table1.username = '$username' or table2.username = '$username'";
$result = mysqli_query($this->connection, $q);

Answer 1

您可以选择两个userid字段，然后查看是否设置了其中一个字段：

$username = "john";

$q = "SELECT table1.userid AS userid1,table2.userid AS userid2 FROM table1,table2 WHERE table1.username = '$username' or table2.username = '$username'";
$result = mysqli_query($this->connection, $q);

Answer 2

试试这个：

$username = "john";

$q = "SELECT table1.userid FROM table1 JOIN table2 ON table1.username=table2.username WHERE table1.username = '$username'";
$result = mysqli_query($this->connection, $q);

如果加工模式（table1.username = table2.username）等于true，则JOIN将table2连接到table1，或者从table1中排除结果中的这一行。

Answer 3

根据我的理解，您想检查一个表中是否已存在该名称。

这是简单的PHP代码：

<?php
 session_start();

 include 'db_connect.php';

 $check_1 = mysqli_query("SELECT * FROM Table_1 WHERE Username = '$Username'");
 $check_2 = mysqli_query("SELECT * FROM Table_2 WHERE Username = '$Username'");

 if(mysqli_num_rows($check_1)==0){
     //Didnt exist in table 1

 }

 else if(mysqli_num_rows($check_2)==0){
     //No result in table 2 either.
 }
 else{
     //what you want to do if the username doesnt exist
 }

让我知道它是如何为您解决的：）

Answer 4

您可以使用UNION子句

$q="SELECT t1.userid FROM table1 as t1 WHERE username='$username' UNION SELECT t2.userid FROM table2 as t2 WHERE username='$username'";

Answer 5

您可以使用左连接

$username = "username";

$q = "SELECT userid 
      FROM table1
      LEFT JOIN table2 ON table1.username = table2.username
      WHERE table1.username = '$username' or table2.username = '$username'";
$result = mysqli_query($this->connection, $q);

如果有或没有通过php查询，如何搜索两个表上的单词

5 个答案: