MySQL从同一查询的两个表中选择

Question

I am trying to select everything from two different mysql tables. I imploded an array called friends so that I can select everything about the user's friends from both tables. Originally I wanted to perform one query on one table and then in a while loop a query on the other. But that didn't work. If you know I way I can nest while loops then please be sure to comment/answer this question.

Here's my code:

$friends = implode("','", $friends);
//implode the friends array for sql query

$sql = mysqli_query($con, "SELECT * FROM users, text_post WHERE username IN ('$friends')") or die(mysqli_error($con));
while ($row = mysqli_fetch_assoc($sql)) {
    echo $row['last_name']. ' ' . $row['body'];
    echo '<br><br>';
}

Note: $row['last_name'] is from the users table and $row['body'] is from the text_post table. I am receiving this error whenever I run my code Column 'username' in where clause is ambiguous

Please help me.

UPDATE:

I changed my query to: $sql = mysqli_query($con, "SELECT * FROM users, text_post WHERE users.username IN ('$friends')") or die(mysqli_error($con)); but now it echo's every match twice:

---

parker hello

parker hello

simms what is up

simms what is up

simms it's raining

simms it's raining

jorge potato

jorge potato

---

Why is it doing that?

The only thing that is the same in both tables is the username.

Answer 1

both tables have a username. specify it using table.username

Answer 2

apparently both tables have the same column, and you are not prefixing the username column with either table name. Maybe what you want is a union?

Answer 3

在where块中使用表前缀users ，即users.username

$sql = mysqli_query($con, "SELECT * FROM users, text_post WHERE users.username IN ('$friends')") or die(mysqli_error($con));

Answer 4

Try this:

$friends = implode("','", $friends);
    //implode the friends array for sql query

    $sql = mysqli_query($con, "SELECT users.last_name,text_post.body FROM users, text_post WHERE username IN ('$friends')") or die(mysqli_error($con));
    while ($row = mysqli_fetch_assoc($sql)) {
        echo $row['last_name']. ' ' . $row['body'];
    }

Answer 5

Your logic would work were it not for username being present in both tables. Because it is, you need to alias your table names so that you can uniquely identify which column you're trying to use in the where clause:

SELECT * from users AS u, text_post as tp WHERE u.username IN(....)

or shorter: SELECT * from users u, text_post tp WHERE u.username IN(....)

As a side note, it's bad practice to SELECT *, you should always explicitly select the columns you want, otherwise your code may crash if you add a column to a table later and forget to update your for loop to include/ignore any changed columns.

Answer 6

Thanks for your question. The issue here is the error that you are receiving of:

Column 'username' in where clause is ambiguous.

What this means is that the statement that you are running is too obscure.

What you need to do to avoid this is to explicitly define the tables and columns that you are attempting to access and thus be implicit in your statements.

As a rule of thumb try and always define your columns in the following format:

{table_name}.{column_name}

With that being said the following should work:

$friends = implode("','", $friends);
/* implode the friends array for sql query. */

$sql = mysqli_query($con, "SELECT * FROM users, text_post WHERE users.username IN ('$friends') AND text_post.username IN ('$friends')") or die(mysqli_error($con));
while ($row = mysqli_fetch_assoc($sql)) {
    echo $row['last_name']. ' ' . $row['body'];
}

Also I would also recommend that you use PHP's PDO Object as appose to the mysqli_query method.

I trust this helps.