connect() Possible reason why the query is successful but cannot be executed

During PHP development, we often encounter this situation: using mysqli_connect() or PDO to successfully connect to the database, but encounter errors when executing SQL queries, such as returning false, no results, and even program crashes. This problem may seem like a problem with database connections at first glance, but the cause may be very complicated in reality. This article will take you to analyze the possible causes of this phenomenon one by one and provide corresponding solutions.

1. Successful connection does not mean everything is normal

 $mysqli = new mysqli("localhost", "user", "password", "database");
if ($mysqli->connect_error) {
    die("Connection failed: " . $mysqli->connect_error);
}
echo "Connection successfully";

Even if the above code successfully outputs "Connection successful", it does not mean that your subsequent query operations will definitely not go wrong. Successful database connection is only a communication channel is established, but whether this channel can handle specific SQL operations normally is affected by many other factors.

2. Common reasons for query failure

1. SQL syntax error

One of the most common reasons is that SQL is written incorrectly. Even if one quote is missing and the field name is spelled incorrectly, it will cause the query to fail.

 $sql = "SELEC * FROM users"; // Wrong keywords SELEC
$result = $mysqli->query($sql);
if (!$result) {
    echo "Query failed: " . $mysqli->error;
}

Solution : Output $mysqli->error to view the error details and carefully check whether the SQL statement is spelled correctly.

2. Insufficient database permissions

Even if the connection is successful, the user may not have permission to perform specific operations, such as reading a certain table, inserting data, etc.

 // If the user has no table SELECT Permissions
$sql = "SELECT * FROM admin_logs";
$result = $mysqli->query($sql);
if (!$result) {
    echo "Permissions问题: " . $mysqli->error;
}

Solution : Check the permissions of the database user and use the following command to view:

 SHOW GRANTS FOR 'user'@'localhost';

If necessary, contact the DBA to activate the permission.

3. The database or table does not exist

If the connected database exists, but attempting to access a table that does not exist, it will also cause the query to fail.

 $sql = "SELECT * FROM nonexistent_table";
$result = $mysqli->query($sql);

Solution : To confirm whether the table exists, you can use the following statement:

 SHOW TABLES LIKE 'nonexistent_table';

4. Character set mismatch

Improper character set settings can cause Chinese or special characters to be inserted fail, and even query exceptions.

 $mysqli->set_charset("utf8mb4"); // Make sure to use the right character set

Solution : Keep the client, connection and database consistent when setting the character set.

5. Parameters are not bound correctly (applicable to preprocessing statements)

When using preprocessing statements, if the parameters are not bound correctly, execution failure will occur:

 $stmt = $mysqli->prepare("SELECT * FROM users WHERE id = ?");
$stmt->bind_param("s", $id); // Incorrect data type or unassigned parameter
$id = 123;
$stmt->execute();

Workaround : Make sure that the variable is correctly initialized before binding the parameters and that the data type matches.

6. Query timeout or connection is interrupted

Some queries run too long and may be automatically interrupted or timed out by the database server.

Solution : Optimize SQL statements to check whether indexes are used, and avoid long-term blocking operations.

3. How to effectively debug query failure

Turn on detailed error report

 mysqli_report(MYSQLI_REPORT_ERROR | MYSQLI_REPORT_STRICT);

This setting throws an exception when an error occurs, making it easier to track problems.

Use logging error

 file_put_contents("log.txt", $mysqli->error . "\n", FILE_APPEND);

Record every error to facilitate analysis of frequency and rules.

Use the packet capture tool to detect requests

If your query contains a URL request, for example, calling a remote interface:

 $url = "https://api.m66.net/getData.php";
$data = file_get_contents($url);

Make sure that the URL is accessible, the return content is correct and not a redirect or a 403 error.

4. Summary

Although the successful connect() indicates that the database connection is normal, "normal connection" does not mean "successful operation". The root cause of query failure is often the permissions, syntax, table structure, character set, or parameter issues. By systematically checking error information, turning on error reports, optimizing query logic and permission configuration, most query failure problems can be effectively solved.

It is important to remember: connection is just the starting point, and query is the key.