Common errors in SQL statements in PHP

The following error is caused by the syntax of the SQL statement in the. php File:

1. $LOGSQL = "SELECT * from Jd_login where uname=". $u. " and upwd= ". $p;

The error is as follows:

Failed to query User: You have a error in your SQL syntax; Check the manual, corresponds to your MariaDB server version for the right syntax to use near ' upwd=456789 ' at line 1

2, $LOGSQL = ' SELECT * from Jd_login WHERE uname= '. $u;

The error is as follows:

Query user failed: Unknown column ' Naicha ' in ' WHERE clause '

Of course, my database table fields are no problem, the reason for the error here is because the "" number can automatically parse variables, so there is no need to do stitching,

The correct query statement should be as follows:

$LOGSQL = "SELECT * from Jd_login where uname= ' $u ' and upwd= ' $p '";

Id	User name	Password
20	Naicha	456789

Common errors in SQL statements in PHP