子查詢
子查詢即嵌套在其它查詢中的查詢。
利用子查詢進行過濾
下面是兩個單獨的查詢語句：
mysql> SELECT order_num FROM orderitems WHERE prod_id = 'TNT2';
+-----------+
| order_num |
+-----------+
| 20005 |
| 20007 |
+-----------+
mysql> SELECT cust_id FROM orders WHERE order_num IN (20005, 20007);
+---------+
| cust_id |
+---------+
| 10001 |
| 10004 |
+---------+
上面兩個子查詢進行組合：
mysql> SELECT cust_id FROM orders WHERE order_num IN (SELECT order_num FROM orderitems WHERE prod_id = 'TNT2');
+---------+
| cust_id |
+---------+
| 10001 |
| 10004 |
+---------+
最終組合后的語句為：
SELECT cust_name, cust_contact
FROM customers
WHERE cust_id IN (SELECT cust_id
FROM orders
WHERE order_num IN (SELECT order_num
FROM orderitems
WHERE prod_id = 'TNT2'));
作為計算字段使用子查詢
mysql> SELECT cust_name,
-> cust_state,
-> (SELECT COUNT(*)
-> FROM orders
-> WHERE orders.cust_id = customers.cust_id) AS orders
-> FROM customers
-> ORDER BY cust_name;
+----------------+------------+--------+
| cust_name | cust_state | orders |
+----------------+------------+--------+
| Coyote Inc. | MI | 2 |
| E Fudd | IL | 1 |
| Mouse House | OH | 0 |
| Wascals | IN | 1 |
| Yosemite Place | AZ | 1 |
+----------------+------------+--------+
聯結表
SQL 最強大的功能之一就是能在做數據檢索查詢的執行中聯結（join）表。
創建聯結（使用外鍵）
mysql> SELECT vend_name, prod_name, prod_price
-> FROM vendors, products
-> WHERE vendors.vend_id = products.vend_id
-> ORDER BY vend_name, prod_name;
+-------------+----------------+------------+
| vend_name | prod_name | prod_price |
+-------------+----------------+------------+
| ACME | Bird seed | 10.00 |
| ACME | Carrots | 2.50 |
| ACME | Detonator | 13.00 |
| ACME | Safe | 50.00 |
| ACME | Sling | 4.49 |
| ACME | TNT (1 stick) | 2.50 |
| ACME | TNT (5 sticks) | 10.00 |
| Anvils R Us | .5 ton anvil | 5.99 |
| Anvils R Us | 1 ton anvil | 9.99 |
| Anvils R Us | 2 ton anvil | 14.99 |
| Jet Set | JetPack 1000 | 35.00 |
| Jet Set | JetPack 2000 | 55.00 |
| LT Supplies | Fuses | 3.42 |
| LT Supplies | Oil can | 8.99 |
+-------------+----------------+------------+
笛卡爾積
由沒有聯結條件的表關系返回的結果為笛卡爾積，檢索出的行的數目將是第一個表中的行數乘以第二個表中的行數。
SELECT vend_name, prod_name, prod_price
FROM vendors, products
ORDER BY vend_name, prod_name;
應該保證所有聯結都有 WHERE 子句。
內部聯結
前面所用的聯結稱為等值聯結，它基于兩個表之間的相等測試。這種聯結也稱為內部聯結，可以使用其它語法實現：
mysql> SELECT vend_name, prod_name, prod_price
-> FROM vendors INNER JOIN products ON vendors.vend_id = products.vend_id;
+-------------+----------------+------------+
| vend_name | prod_name | prod_price |
+-------------+----------------+------------+
| Anvils R Us | .5 ton anvil | 5.99 |
| Anvils R Us | 1 ton anvil | 9.99 |
| Anvils R Us | 2 ton anvil | 14.99 |
| LT Supplies | Fuses | 3.42 |
| LT Supplies | Oil can | 8.99 |
| ACME | Detonator | 13.00 |
| ACME | Bird seed | 10.00 |
| ACME | Carrots | 2.50 |
| ACME | Safe | 50.00 |
| ACME | Sling | 4.49 |
| ACME | TNT (1 stick) | 2.50 |
| ACME | TNT (5 sticks) | 10.00 |
| Jet Set | JetPack 1000 | 35.00 |
| Jet Set | JetPack 2000 | 55.00 |
+-------------+----------------+------------+
SQL 規范推薦使用 INNER JOIN。
創建高級聯結
使用表別名
使用表別名的理由為：
縮短 SQL 語句
允許在單條 SELECT 語句中多次使用相同的表
SELECT cust_name, cust_contact
FROM customers AS c, orders AS o, orderitems as oi
WHERE c.cust_id = o.cust_id
AND oi.order_num = o.order_num
AND prod_id = 'TNT2';
自聯結
查詢 prod_id 為 DTNTR 產品所屬的供應商的所有產品：
mysql> SELECT prod_id, prod_name
-> FROM products
-> WHERE vend_id = (SELECT vend_id
-> FROM products
-> WHERE prod_id = 'DTNTR');
+---------+----------------+
| prod_id | prod_name |
+---------+----------------+
| DTNTR | Detonator |
| FB | Bird seed |
| FC | Carrots |
| SAFE | Safe |
| SLING | Sling |
| TNT1 | TNT (1 stick) |
| TNT2 | TNT (5 sticks) |
+---------+----------------+
使用自聯結：
mysql> SELECT p1.prod_id, p1.prod_name
-> FROM products AS p1, products AS p2
-> WHERE p1.vend_id = p2.vend_id
-> AND p2.prod_id = 'DTNTR';
+---------+----------------+
| prod_id | prod_name |
+---------+----------------+
| DTNTR | Detonator |
| FB | Bird seed |
| FC | Carrots |
| SAFE | Safe |
| SLING | Sling |
| TNT1 | TNT (1 stick) |
| TNT2 | TNT (5 sticks) |
+---------+----------------+
自聯結通常作為外部語句用來替代從相同表中檢索數據時使用的子查詢語句，雖然最終的結果是相同的，但有時候處理聯結遠比處理子查詢快的多。
自然聯結
SELECT c.*, o.order_num, o.order_date,
oi.prod_id, oi.quantity, oi.item_price
FROM customers AS c, orders AS o, orderitems AS oi
WHERE c.cust_id = o.cust_id
AND oi.order_num = o.order_num
AND prod_id = 'FB'
外部聯結
檢索所有訂單的客戶：
mysql> SELECT customers.cust_id, orders.order_num
-> FROM customers INNER JOIN orders
-> ON customers.cust_id = orders.cust_id;
+---------+-----------+
| cust_id | order_num |
+---------+-----------+
| 10001 | 20005 |
| 10001 | 20009 |
| 10003 | 20006 |
| 10004 | 20007 |
| 10005 | 20008 |
+---------+-----------+
使用外部聯結，檢索所有客戶，包括那些沒訂單的。
mysql> SELECT customers.cust_id, orders.order_num
-> FROM customers LEFT OUTER JOIN orders
-> ON customers.cust_id = orders.cust_id;
+---------+-----------+
| cust_id | order_num |
+---------+-----------+
| 10001 | 20005 |
| 10001 | 20009 |
| 10002 | NULL |
| 10003 | 20006 |
| 10004 | 20007 |
| 10005 | 20008 |
+---------+-----------+
存在兩種基本的外部聯結形式：左外部聯結和右外部聯結，左聯結以左為主，右聯結以右為主，沒有補 null。
使用帶聚合函數的聯結
mysql> SELECT customers.cust_name, customers.cust_id, COUNT(orders.order_num) AS num_ord
-> FROM customers LEFT JOIN orders ON customers.cust_id = orders.cust_id
-> GROUP BY customers.cust_id;
+----------------+---------+---------+
| cust_name | cust_id | num_ord |
+----------------+---------+---------+
| Coyote Inc. | 10001 | 2 |
| Mouse House | 10002 | 0 |
| Wascals | 10003 | 1 |
| Yosemite Place | 10004 | 1 |
| E Fudd | 10005 | 1 |
+----------------+---------+---------+
