MariaDB Join语句
MariaDB Join语句
在前面的示例中,我们都是从单个表中检索数据,但是大多数项目的数据操作要复杂得多,需要从多个表中进行聚合、比较和检索。
JOIN允许将两个或多个表合并成一个查询对象。它们通过选择、更新和删除语句来使用。
查看JOIN语句的基本语法,如下所示:
SELECT column FROM table_name1 INNER JOIN table_name2 ON table_name1.column = table_name2.column;
注意JOIN的旧语法使用隐式连接,没有关键字。可以使用WHERE子句来实现连接,但是关键字在可读性、维护和最佳实践方面最有效。
JOIN有多种形式,如左连接(left join)、右连接(right join)或内连接(inner join)。基于共享值或特性,各种连接类型提供不同类型的聚合。
我们可以在命令提示符或使用PHP脚本使用JOIN。
命令提示符
在命令提示符下,只需使用标准语句:
root@host# mysql -u root -p password; Enter password:******* mysql> use PRODUCTS; Database changed mysql> SELECT products.ID_number, products.Nomenclature, inventory.inventory_ct FROM products INNER JOIN inventory ON products.ID_numbeer = inventory.ID_number; +-------------+----------------+-----------------+ | ID_number | Nomenclature | Inventory Count | +-------------+----------------+-----------------+ | 12345 | Orbitron 4000 | 150 | +-------------+----------------+-----------------+ | 12346 | Orbitron 3000 | 200 | +-------------+----------------+-----------------+ | 12347 | Orbitron 1000 | 0 | +-------------+----------------+-----------------+
PHP使用JOIN
php使用mysql_query() 方法执行JOIN:
<?php $dbhost = 'localhost:3036'; $dbuser = 'root'; $dbpass = 'rootpassword'; $conn = mysql_connect($dbhost, $dbuser, $dbpass); if(! $conn ) { die('Could not connect: ' . mysql_error()); } $sql = 'SELECT a.product_id, a.product_manufacturer, b.product_count FROM products_tbl a, pcount_tbl b WHERE a.product_manufacturer = b.product_manufacturer'; mysql_select_db('PRODUCTS'); $retval = mysql_query( $sql, $conn ); if(! $retval ) { die('Could not get data: ' . mysql_error()); } while($row = mysql_fetch_array($retval, MYSQL_ASSOC)) { echo "Manufacturer:{$row['product_manufacturer']} <br> ". "Count: {$row['product_count']} <br> ". "Product ID: {$row['product_id']} <br> ". "--------------------------------<br>"; } echo "Fetched data successfully\n"; mysql_close($conn); ?>
成功检索数据后,您将看到以下输出:
ID Number: 12345 Nomenclature: Orbitron 4000 Inventory Count: 150 -------------------------------------- ID Number: 12346 Nomenclature: Orbitron 3000 Inventory Count: 200 -------------------------------------- ID Number: 12347 Nomenclature: Orbitron 1000 Inventory Count: 0 -------------------------------------- mysql> Fetched data successfully