PHP如何用if语句按优先级查询三个同结构表的指定ID数据

你原有代码用逗号拼接多表查询会生成笛卡尔积，既不符合逐表判断优先级的要求，还存在直接拼接用户传入参数导致的SQL注入风险。以下是通过if语句实现优先级判断的正确代码：

核心逻辑 #

• 优先查最高优先级的table1，匹配到对应ID记录就直接提取数据，不再执行后续表查询
• table1无匹配时再查table2，匹配到记录就取数终止流程
• 前两张表都没有匹配记录时，最后查询table3取数
• 增加参数校验、查询结果判断和三表无匹配的兜底逻辑，避免报错和安全漏洞

<?php
include('config/database.php');
// 校验传入id参数，转为整型规避SQL注入，适用于id为数字类型的场景
$id = isset($_GET['single']) ? intval($_GET['single']) : 0;
// 初始化存储结果的变量
$title = $date = $admin = $thumbnail = $description = $category = '';
$found = false;

// 第一步：查询最高优先级的table1
$query = mysqli_query($conn, "SELECT * FROM `table1` WHERE `id` = {$id} LIMIT 1");
if ($query && mysqli_num_rows($query) == 1) {
    $row = mysqli_fetch_assoc($query);
    $title = $row['title'];
    $date = $row['date'];
    $admin = $row['admin'];
    $thumbnail = $row['thumbnail'];
    $description = $row['description'];
    $category = $row['category'];
    $found = true;
}

// 第二步：table1未查到则查询table2
if (!$found) {
    $query = mysqli_query($conn, "SELECT * FROM `table2` WHERE `id` = {$id} LIMIT 1");
    if ($query && mysqli_num_rows($query) == 1) {
        $row = mysqli_fetch_assoc($query);
        $title = $row['title'];
        $date = $row['date'];
        $admin = $row['admin'];
        $thumbnail = $row['thumbnail'];
        $description = $row['description'];
        $category = $row['category'];
        $found = true;
    }
}

// 第三步：前两个表都未查到则查询table3
if (!$found) {
    $query = mysqli_query($conn, "SELECT * FROM `table3` WHERE `id` = {$id} LIMIT 1");
    if ($query && mysqli_num_rows($query) == 1) {
        $row = mysqli_fetch_assoc($query);
        $title = $row['title'];
        $date = $row['date'];
        $admin = $row['admin'];
        $thumbnail = $row['thumbnail'];
        $description = $row['description'];
        $category = $row['category'];
        $found = true;
    }
}

// 三表均无匹配的兜底逻辑
if (!$found) {
    echo "未找到对应ID的记录";
}

// 后续可直接使用$title、$date等提取到的变量
?>

补充说明 #

• 代码中加LIMIT 1是因为ID通常为表主键，单表内唯一，只查1条可以提升查询效率
• 如果你的id是字符串类型，把参数校验部分改成$id = mysqli_real_escape_string($conn, $_GET['single']);做转义即可，禁止直接拼接未处理的用户输入到SQL语句中
• 整套逻辑完全通过if语句实现优先级判断，匹配到高优先级表的记录后不会再查询低优先级表，完全符合需求

内容的提问来源于stack exchange，提问作者Liton Kumar