[mySQL]关于add table的一个注意小事项，直接影响后续populate table

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<!DOCTYPE html><html lang="en"><head>    <meta charset="utf-8">    <meta http-equiv="X-UA-Compatible" content="IE=edge">    <meta name="viewport" content="width=device-width, initial-scale=1">    <title>Add a table to database</title>    <link href="css/bootstrap.min.css" rel="stylesheet">    <style>        h1 {            color: purple;        }                h3 {            color: #42d5ce;        }                .containingDiv {            border: 1px solid #7c73f6;            margin-top: 100px;            border-radius: 15px;        }    </style></head><body>    <div class="container-fluid">        <div class="row">            <div class="col-sm-offset-1 col-sm-10 containingDiv">                <h1>Add a table to database:</h1>                <h3>Connect to database</h3>                <?php                    //mysqli_connect(host, username, password, dbname)                    $link = @mysqli_connect("localhost", "cdwccdwc_user", "bobo1989..", "cdwccdwc_users") or die("ERROR: Unable to connect: " . mysqli_connect_error());                    echo "<p>Connected successfully to the database.</p>";                    ?>                                <h3>Create table</h3>                <?php                    $sql = "CREATE TABLE users3(ID INT(4) NOT NULL PRIMARY KEY AUTO_INCREMENT, firstname CHAR(20) NOT NULL, lastname CHAR(20) NOT NULL, email VARCHAR(30), password VARCHAR(40))";//AUTO_INCREMENT is very important,because if not add it, you can just insert all the values only once.                    if(mysqli_query($link, $sql)){                        echo "<p>Table users created successfuly!</p>";                       }else{                        echo "ERROR: Unable to execute $sql" . mysqli_error($link);                       }                ?>            </div>        </div>    </div>    <script src="https://ajax.googleapis.com/ajax/libs/jquery/1.11.2/jquery.min.js"></script>    <script src="js/bootstrap.min.js"></script></body></html>

如图，红色加粗标记的AUTO_INCREMENT,在创建table的时候加与不加的区别在于：

加AUTO_INCREMENT：后续populate table 的时候可以添加任意数量的表行内容，因为entry for key "primary"在自动增加

不加AUTO_INCREMENT：上面操作只能添加一次，会报错，例如：ERROR: Unable to excecute: INSERT INTO users3 (firstname, lastname, email, password) VALUES ('tom', 'cluz', 'tm@gmial.com', 'ef0c0fd6710ab42cd8595bee67c65424'). Duplicate entry '0' for key 'PRIMARY'。

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