MySQL基礎部分總結

摘要： MySQL 1、選擇資料庫 use dbname show databases; 2、資料表 show tables mysql> show columns from customers; mysql> desc c...

MySQL

1、選擇資料庫

1. use dbname
2. show databases;

2、資料表

1. show tables
2. mysql> show columns from customers;
3. mysql> desc customers;

3、show 語句

1. show status
2. show create databases
3. show create table
4. show grants

4、select 檢索

4.1.1版本後不再區分大小寫，但是為了容易閱讀和除錯，建議還是使用。

mysql> select cust_name from customers;
mysql> select cust_name cust_status from customers;
mysql> select distinct vend_id from products;
mysql> select prod_name from products limit 5;
mysql> select prod_name from products limit 5,5;
//分頁問題
從行0開始計數，limit5,5是從第5行開始（不包括第五行），取5行，結果是：6:10行
因為這個很容易引起誤解，所以MySQL5開始支援另一個語法：limit 4 offset 3，意思是從行3開始取4行，等同於limit 3,4

4-1、排序資料

//單個欄位排序
mysql> select prod_name from products order byprod_name;
//多個欄位排序，如果第一個欄位全部唯一則第二個欄位就不會生效
mysql> select prod_id,prod_price,prod_name from products order by prod_price ,prod_name;

4-2、指定排序方向

• desc 降序
• asc 升序-預設

注意順序，from>ordrr by >limit

mysql> select prod_id,prod_price,prod_name from products order by prod_price desc;
mysql> select prod_id,prod_price,prod_name from products order by prod_price asc;
mysql> select prod_price from products order by prod_price desc limit 1;

5、where 條件

相關操作符：

1. = 等於
2. <> 不等於
3. != 不等於
4. < 小於
5. > 大於
6. >= 大於或者等於
7. <= 小於或等於
8. between 兩者之間 and

and 的優先順序大於or，需要處理or，則需要括號

mysql> select prod_price,prod_name fromproducts where prod_price = 2.50;
mysql> select prod_price,prod_name fromproducts where prod_pricebetween 5 and 10;
//IS NULL
mysql> select cust_id from customers where cust_email is null;

重點：空值檢查

空值既是：NULL

MySQL中判斷是否是空值的子句是：IS NULL

example:

mysql> select cust_id FROM customerswhere cust_email IS NULL;
+---------+
| cust_id |
+---------+
|10002 |
|10005 |
+---------+

6、where 資料過濾

（logical operator）邏輯操作符：and-or

mysql> select prod_id,prod_price,prod_name from products where vend_id = 1003 and prod_price<= 10;
mysql> select prod_id,prod_price,prod_name from products where vend_id = 1003 or vend_id = 1002;

運算子優先順序問題：

下列SQL中實際先執行vend_id = 1002 and prod_price >= 10; ,再執行vend_id = 1003 .因為and的優先順序大於or，如果要按理想執行，加括號！

mysql> select prod_id,prod_price,prod_name from products where vend_id = 1003 or vend_id = 1002 and prod_price >= 10;
mysql> select prod_id,prod_price,prod_name from products where (vend_id = 1003 or vend_id = 1002 )and prod_price >= 10;

6-1、 in操作符 (not in)

mysql> select prod_id,prod_price,prod_name from products where vend_id in (1002,1003) order by prod_name;

6-2、 or操作符

mysql> select prod_id,prod_price,prod_name from products where vend_id not in (1002,1003) order by prod_name;

7、用萬用字元過濾

like 和 _ 的區別是後者只能匹配一個字元

7-1、like

**注意NULL 雖然似乎 % 萬用字元可以匹配任何東西，但有一個例

外，即 NULL 。即使是 WHERE prod_name LIKE '%' 也不能匹配

用值 NULL 作為產品名的行。**

mysql> select prod_id,prod_price,prod_name from products where prod_nameLIKE 'jet%';

mysql> select prod_id,prod_price,prod_name from products where prod_nameLIKE '%anv%';

7-2、_

mysql> select prod_id,prod_price,prod_name from products where prod_nameLIKE '_ ton anvil';

8、正則表示式

like是匹配全部，REGEXP可以匹配全部和部分

mysql> select prod_namefrom products where prod_name ='JetPack 1000';
+--------------+
| prod_name|
+--------------+
| JetPack 1000 |
+--------------+
1 row in set (0.00 sec)

mysql> select prod_name from products where prod_nameREGEXP '1000';
+--------------+
| prod_name|
+--------------+
| JetPack 1000 |
+--------------+
1 row in set (0.00 sec)

預設不區分大小寫，需要區分大小寫binary

mysql> select prod_name from products where prod_nameREGEXP 'jetpack .000';
mysql> select prod_name from products where prod_nameREGEXP binary 'JetPack .000';

10、計算欄位

1. concat 合併講兩個欄位合併成一個新的欄位

mysql> select concat (vend_name , 'C',vend_country,')') from vendors order by vend_name;
+-------------------------------------------+
| concat (vend_name , 'C',vend_country,')') |
+-------------------------------------------+
| ACMECUSA)|
| Anvils R UsCUSA)|
| Furball Inc.CUSA)|
| Jet SetCEngland)|
| Jouets Et OursCFrance)|
| LT SuppliesCUSA)|
+-------------------------------------------+
6 rows in set (0.00 sec)

1. rtrim (ltrim ,trim) 去掉空格

mysql> select concat (rtrim(vend_name) , 'C',vend_country,')') from vendors order by vend_name;

1. as 別名

mysql> select concat (rtrim(vend_name) , '(',rtrim(vend_country),')') as vend_titlefrom vendors order by vend__name;

1. 計算

+、-、* 、\

mysql> select quantity*item_price as expand_price from orderitems where order_num =20005;

11、函式

1. trim、ltrim、rtrim 去掉空值
2. Upper 轉為大寫

mysql> select vend_name,upper(vend_name) as ven_name_upcase from vendors order by vend_name;

11-2 時間函式

1. AddDate() 增加一個日期(天、周等)
2. AddTime() 增加一個時間(時、分等)
3. CurDate() 返回當前日期
4. CurTime() 返回當前時間
5. ==Date() 返回日期時間的日期部分==
6. DateDiff() 計算兩個日期之差
7. Date_Add() 高度靈活的日期運算函式
8. Date_Format() 返回一個格式化的日期或時間串
9. Day() 返回一個日期的天數部分
10. DayOfWeek() 對於一個日期，返回對應的星期幾
11. Hour() 返回一個時間的小時部分
12. Minute() 返回一個時間的分鐘部分
13. Month() 返回一個日期的月份部分
14. Now() 返回當前日期和時間
15. Second() 返回一個時間的秒部分
16. Time() 返回一個日期時間的時間部分
17. Year() 返回一個日期的年份部分

取9月某一天的資料

mysql> select cust_id,order_num from orders where Date(order_date) = '2005-09-01'; 
+---------+-----------+
| cust_id | order_num |
+---------+-----------+
|10001 |20005 |
+---------+-----------+
1 row in set (0.00 sec)

取9月整個月的訂單

mysql> select cust_id,order_num from orders where Date(order_date)between '2005-09-01' and '2005-09-30'; 
+---------+-----------+
| cust_id | order_num |
+---------+-----------+
|10001 |20005 |
|10003 |20006 |
|10004 |20007 |
+---------+-----------+
3 rows in set (0.00 sec)

mysql> select cust_id,order_num from orders where Year(order_date) and month(order_date) = 9;
+---------+-----------+
| cust_id | order_num |
+---------+-----------+
|10001 |20005 |
|10003 |20006 |
|10004 |20007 |
+---------+-----------+
3 rows in set (0.00 sec)

11-4 數值處理函式

1. Abs() 返回一個數的絕對值
2. Cos() 返回一個角度的餘弦
3. Exp() 返回一個數的指數值
4. Mod() 返回除操作的餘數
5. Pi() 返回圓周率
6. Rand() 返回一個隨機數
7. Sin() 返回一個角度的正弦
8. Sqrt() 返回一個數的平方根
9. Tan() 返回一個角度的正切

11-5 聚集函式

1. AVG() 返回某列的平均值
2. COUNT() 返回某列的行數
3. MAX() 返回某列的最大值
4. MIN() 返回某列的最小值
5. SUM() 返回某列值之和
6. DISTINCT

mysql> select avg(prod_price) as avg_price from products;

分組資料

GROUP BY子句和HAVING子句

mysql> select vend_id,count(*) as num_prods from products group by vend_id; 
+---------+-----------+
| vend_id | num_prods |
+---------+-----------+
|1001 |3 |
|1002 |2 |
|1003 |7 |
|1005 |2 |
+---------+-----------+
4 rows in set (0.00 sec)

mysql> select vend_id,count(*) as num_prods from products group by vend_id with rollup;
+---------+-----------+
| vend_id | num_prods |
+---------+-----------+
|1001 |3 |
|1002 |2 |
|1003 |7 |
|1005 |2 |
|NULL |14 |
+---------+-----------+
5 rows in set (0.00 sec)

having

mysql> select vend_id,count(*) as num_prods from products group by vend_id having count(*)>=2;
+---------+-----------+
| vend_id | num_prods |
+---------+-----------+
|1001 |3 |
|1002 |2 |
|1003 |7 |
|1005 |2 |
+---------+-----------+
4 rows in set (0.00 sec)


mysql> select vend_id,count(*) as num_prods from products where prod_price>=10group by vend_id having count(*)>=2; 
+---------+-----------+
| vend_id | num_prods |
+---------+-----------+
|1003 |4 |
|1005 |2 |
+---------+-----------+
2 rows in set (0.00 sec)

mysql> select order_num ,sum(quantity*item_price) as ordertotal from orderitems 
-> group by order_num
-> having sum(quantity*item_price) >= 50
-> order by ordertotal;
+-----------+------------+
| order_num | ordertotal |
+-----------+------------+
|20006 |55.00 |
|20008 |125.00 |
|20005 |149.87 |
|20007 |1000.00 |
+-----------+------------+
4 rows in set (0.00 sec)

順序

• select
• from
• where
• group by
• having
• order by
• limit

12 子查詢

mysql>select cust_id from orders where order_num in(select order_num from orderitems where prod_id ='TNT2');
+---------+
| cust_id |
+---------+
|10001 |
|10004 |
+---------+

15 連線表

笛卡兒積(cartesian product)

如果將兩個表同時作為資料來源(from後的表名)，不加任何的匹配條件，那麼產生的結果集就是一個迪卡爾積。

迪卡爾積的結果沒有意義，但是迪卡爾積是聯合查詢、連線查詢的基礎。

1. 交叉連線 cross join

使用表A中的1條記錄去表B中連線所有的記錄，就是笛卡爾積

2. 內連線

select 欄位列表 from 表A 【inner】 join 表B ，匹配到的成功的記錄

3. 外連線分為左連線和右連線，

左連線保留左邊的所有，右邊匹配到的部分

4. using關鍵字

在進行連線時，如果進行連線的兩個欄位的名子相同，則可以使用using using('cid')

當前筆記出自 《MySQL必知必會》