php - 在插入数据之前检查现有条目

Question

只有当新客户不存在时，我才尝试插入新客户。我认为 SQL 语句有问题，但我也想在 PHP 中提供上下文:

   $sql = "INSERT INTO clients
        (`studentEmail`, `studentPassword`, `parentEmail`, `parentPassword`,
        `studentFirstName`, `studentLastName`, `studentPhone`, `parentFirstName`,
        `parentLastName`, `parentPhone`, `school`)
         VALUES ('$studentEmail', '$studentPassword', '$parentEmail',
        '$parentPassword', '$studentFirstName', '$studentLastName',
        '$studentPhone', '$parentFirstName', '$parentLastName', '$parentPhone', '$school')
        SELECT studentEmail
        FROM clients
        WHERE not exists (select * from clients where studentEmail == '"$studentEmail"')";

Answer 1

在插入语句中，select 和values 是互斥的。而不是:

insert table1 (col1, col2, col3, ...)
values (1, 2, 3, ...)
select col1
where  not exists (...)

尝试:

insert table1 (col1, col2, col3, ...)
select 1, 2, 3, ...
where  not exists (...)