mysql_fetch_field 和mysql_fetch_assoc 有什麼區別？教你如何根據需求選擇函數

gitbox 2025-08-26

1. 概念區別（一句話總結）

mysql_fetch_assoc() ：（鍵名為字段名，值為該行對應字段的值）。常用於遍歷查詢結果的每一行數據。
mysql_fetch_field() ：從結果集取得字段（列）元信息（例如字段名、類型、長度等），用於獲取列的結構信息，而不是行數據。

2. 典型用途對比

mysql_fetch_assoc()用於：讀取查詢返回的數據行，例如顯示用戶列表、處理每條記錄。
mysql_fetch_field()用於：讀取查詢結果集的字段定義，例如在動態生成表頭、自動生成表單、檢查列類型或長度時使用。

3. 例子（ mysql擴展，說明用途— 僅示範，注意已棄用）

 <span><span><span class="hljs-comment">// 假設已有 $link = mysql_connect(...); mysql_select_db(...);</span></span><span>

</span><span><span class="hljs-comment">// 查詢數據</span></span><span>
</span><span><span class="hljs-variable">$res</span></span><span> = </span><span><span class="hljs-title function_ invoke__">mysql_query</span></span><span>(</span><span><span class="hljs-string">"SELECT id, username, email FROM users"</span></span><span>);

</span><span><span class="hljs-comment">// 使用 mysql_fetch_assoc() 遍歷每一行</span></span><span>
</span><span><span class="hljs-keyword">while</span></span><span> (</span><span><span class="hljs-variable">$row</span></span><span> = </span><span><span class="hljs-title function_ invoke__">mysql_fetch_assoc</span></span><span>(</span><span><span class="hljs-variable">$res</span></span><span>)) {
    </span><span><span class="hljs-comment">// $row 是關聯數組： ['id' =&gt; '1', 'username' =&gt; 'alice', 'email' =&gt; '[email protected]']</span></span><span>
    </span><span><span class="hljs-keyword">echo</span></span><span> </span><span><span class="hljs-string">"ID: <span class="hljs-subst">{$row['id']}</span></span></span><span>, 使用者名稱: </span><span><span class="hljs-subst">{$row['username']}</span></span><span>\n";
}

</span><span><span class="hljs-comment">// 獲取字段信息（例如1。2列的信息）</span></span><span>
</span><span><span class="hljs-title function_ invoke__">mysql_data_seek</span></span><span>(</span><span><span class="hljs-variable">$res</span></span><span>, </span><span><span class="hljs-number">0</span></span><span>); </span><span><span class="hljs-comment">// 可選：定位指針（mysql_fetch_field 不改變行指針，但與使用習慣有關）</span></span><span>
</span><span><span class="hljs-variable">$field</span></span><span> = </span><span><span class="hljs-title function_ invoke__">mysql_fetch_field</span></span><span>(</span><span><span class="hljs-variable">$res</span></span><span>, </span><span><span class="hljs-number">1</span></span><span>); </span><span><span class="hljs-comment">// 參數 1 表示1。 2 列（索引從 0 開始）</span></span><span>
</span><span><span class="hljs-keyword">if</span></span><span> (</span><span><span class="hljs-variable">$field</span></span><span>) {
    </span><span><span class="hljs-keyword">echo</span></span><span> </span><span><span class="hljs-string">"1。2列名: <span class="hljs-subst">{$field-&gt;name}</span></span></span><span>, 類型: </span><span><span class="hljs-subst">{$field-&gt;type}</span></span><span>, 長度: </span><span><span class="hljs-subst">{$field-&gt;max_length}</span></span><span>\n";
}
</span></span>

說明：

mysql_fetch_assoc()返回一個數組用於訪問行數據。
mysql_fetch_field()返回一個對象（或false ）包含列的元信息（name, orgname, max_length, type 等）。

4. 常見誤區

誤以為mysql_fetch_field()會返回某一行的字段值——不是，它返回字段的結構/元信息。要取值請用mysql_fetch_assoc() / mysql_fetch_row()等。
認為mysql_fetch_assoc()能取得字段的類型或長度——不能，它只提供值（和字段名作為鍵）。

5. 選擇建議（什麼時候用哪個）

你要遍歷查詢結果並處理數據：用mysql_fetch_assoc() （或更現代的mysqli_fetch_assoc() / PDO 的fetch(PDO::FETCH_ASSOC) ）。
你要動態生成表頭、驗證列類型或讀取列名/長度等元信息：用mysql_fetch_field() （或mysqli_fetch_field_direct() 、 $result->fetch_field() 、PDO 的getColumnMeta() （注意兼容性和限制））。
如果目標是兼顧安全、性能與長遠維護，請盡快遷移到mysqli或PDO 。 mysql擴展在PHP 7+ 被移除。

6. 推薦的現代替代（ mysqli與PDO示例）

mysqli（面向過程）

 <span><span><span class="hljs-variable">$mysqli</span></span><span> = </span><span><span class="hljs-title function_ invoke__">mysqli_connect</span></span><span>(</span><span><span class="hljs-string">'host'</span></span><span>,</span><span><span class="hljs-string">'user'</span></span><span>,</span><span><span class="hljs-string">'pass'</span></span><span>,</span><span><span class="hljs-string">'db'</span></span><span>);
</span><span><span class="hljs-variable">$res</span></span><span> = </span><span><span class="hljs-title function_ invoke__">mysqli_query</span></span><span>(</span><span><span class="hljs-variable">$mysqli</span></span><span>, </span><span><span class="hljs-string">"SELECT id, username, email FROM users"</span></span><span>);

</span><span><span class="hljs-comment">// 取行數據（關聯數組）</span></span><span>
</span><span><span class="hljs-keyword">while</span></span><span> (</span><span><span class="hljs-variable">$row</span></span><span> = </span><span><span class="hljs-title function_ invoke__">mysqli_fetch_assoc</span></span><span>(</span><span><span class="hljs-variable">$res</span></span><span>)) {
    </span><span><span class="hljs-keyword">echo</span></span><span> </span><span><span class="hljs-variable">$row</span></span><span>[</span><span><span class="hljs-string">'username'</span></span><span>] . </span><span><span class="hljs-string">"\n"</span></span><span>;
}

</span><span><span class="hljs-comment">// 取字段信息（1。 1 列）</span></span><span>
</span><span><span class="hljs-variable">$fieldInfo</span></span><span> = </span><span><span class="hljs-title function_ invoke__">mysqli_fetch_field_direct</span></span><span>(</span><span><span class="hljs-variable">$res</span></span><span>, </span><span><span class="hljs-number">1</span></span><span>); </span><span><span class="hljs-comment">// 或 mysqli_fetch_field($res) 在遍歷字段時</span></span><span>
</span><span><span class="hljs-keyword">if</span></span><span> (</span><span><span class="hljs-variable">$fieldInfo</span></span><span>) {
    </span><span><span class="hljs-keyword">echo</span></span><span> </span><span><span class="hljs-variable">$fieldInfo</span></span><span>-&gt;name . </span><span><span class="hljs-string">' / '</span></span><span> . </span><span><span class="hljs-variable">$fieldInfo</span></span><span>-&gt;type . </span><span><span class="hljs-string">"\n"</span></span><span>;
}
</span></span>

PDO

 <span><span><span class="hljs-variable">$pdo</span></span><span> = </span><span><span class="hljs-keyword">new</span></span><span> </span><span><span class="hljs-title function_ invoke__">PDO</span></span><span>(</span><span><span class="hljs-string">'mysql:host=host;dbname=db;charset=utf8mb4'</span></span><span>, </span><span><span class="hljs-string">'user'</span></span><span>, </span><span><span class="hljs-string">'pass'</span></span><span>);
</span><span><span class="hljs-variable">$stmt</span></span><span> = </span><span><span class="hljs-variable">$pdo</span></span><span>-&gt;</span><span><span class="hljs-title function_ invoke__">query</span></span><span>(</span><span><span class="hljs-string">"SELECT id, username, email FROM users"</span></span><span>);

</span><span><span class="hljs-comment">// 取行數據</span></span><span>
</span><span><span class="hljs-keyword">while</span></span><span> (</span><span><span class="hljs-variable">$row</span></span><span> = </span><span><span class="hljs-variable">$stmt</span></span><span>-&gt;</span><span><span class="hljs-title function_ invoke__">fetch</span></span><span>(PDO::</span><span><span class="hljs-variable constant_">FETCH_ASSOC</span></span><span>)) {
    </span><span><span class="hljs-keyword">echo</span></span><span> </span><span><span class="hljs-variable">$row</span></span><span>[</span><span><span class="hljs-string">'email'</span></span><span>] . </span><span><span class="hljs-string">"\n"</span></span><span>;
}

</span><span><span class="hljs-comment">// 取字段元信息（注意：PDO::getColumnMeta 在某些驅動/版本上不穩定）</span></span><span>
</span><span><span class="hljs-variable">$meta</span></span><span> = </span><span><span class="hljs-variable">$stmt</span></span><span>-&gt;</span><span><span class="hljs-title function_ invoke__">getColumnMeta</span></span><span>(</span><span><span class="hljs-number">1</span></span><span>); </span><span><span class="hljs-comment">// 1。 2 列</span></span><span>
</span><span><span class="hljs-keyword">if</span></span><span> (</span><span><span class="hljs-variable">$meta</span></span><span>) {
    </span><span><span class="hljs-keyword">echo</span></span><span> </span><span><span class="hljs-variable">$meta</span></span><span>[</span><span><span class="hljs-string">'name'</span></span><span>] . </span><span><span class="hljs-string">', native_type='</span></span><span> . (</span><span><span class="hljs-variable">$meta</span></span><span>[</span><span><span class="hljs-string">'native_type'</span></span><span>] ?? </span><span><span class="hljs-string">'unknown'</span></span><span>) . </span><span><span class="hljs-string">"\n"</span></span><span>;
}
</span></span>

7. 小結（快速回顧）

mysql_fetch_assoc() ：返回行數據（關聯數組）—— 日常讀取記錄首選。
mysql_fetch_field() ：返回列元信息（對象）—— 用於讀取列名、類型、長度等結構性信息。
實際開發中應使用mysqli或PDO ，並用對應的fetch_assoc() / fetch_field() / getColumnMeta()等替代函數來完成相同的任務。

8. 額外建議

盡量使用參數化查詢（預處理語句）以防SQL 注入（ mysqli的prepare / bind_param ，或PDO 的prepare / bindValue ）。
如果只是需要列名列表，一種簡單做法是取第一行的關聯數組並用array_keys()獲取字段名（注意當結果集為空時需額外處理）。