从零开始数据库阶段一（二）

1. 预备阶段

　　索引的概念不是很难，但是想要建立高效的索引还是需要经验的，然后再进行下面的学习的时候先回顾了数据库索引、mysql索引知识点汇总、DSAA补充之B树、B+树。以下引用来自runoob。

2. LIKE

• like 匹配/模糊匹配，会与 % 和 _ 结合使用。
• SQL原语：SELECT column_name(s) FROM table_name WHERE column_name LIKE pattern;
• like后面没有与%或者_结合时，效果和=一样

'%a'     //以a结尾的数据
'a%'     //以a开头的数据
'%a%'    //含有a的数据
'_a_'    //三位且中间字母是a的
'_a'     //两位且结尾字母是a的
'a_'     //两位且开头字母是a的　

　　根据昨天的学习迅速建立一个数据表，并实践上面的结论：

MariaDB [dh]> CREATE TABLE dh_tl( id INT AUTO_INCREMENT, name VARCHAR(255) NOT NULL, goal INT NOT NULL, PRIMARY KEY(id)); 
Query OK, 0 rows affected (0.14 sec)
MariaDB [dh]> INSERT INTO dh_tl (name,goal) VALUES ('abc',100),('bcd',99),('cdf',98);     
Query OK, 3 rows affected (0.02 sec)
Records: 3  Duplicates: 0  Warnings: 0
MariaDB [dh]> SELECT * FROM dh_tl;
+----+------+------+
| id | name | goal |
+----+------+------+
|  1 | abc  |  100 |
|  2 | bcd  |   99 |
|  3 | cdf  |   98 |
+----+------+------+
3 rows in set (0.00 sec)
MariaDB [dh]> SELECT * FROM dh_tl WHERE name LIKE '%c';
+----+------+------+
| id | name | goal |
+----+------+------+
|  1 | abc  |  100 |
+----+------+------+
1 row in set (0.00 sec)

MariaDB [dh]> SELECT * FROM dh_tl WHERE name LIKE 'c%'; 
+----+------+------+
| id | name | goal |
+----+------+------+
|  3 | cdf  |   98 |
+----+------+------+
1 row in set (0.00 sec)

MariaDB [dh]> SELECT * FROM dh_tl WHERE name LIKE 'c_';
Empty set (0.00 sec)

MariaDB [dh]> SELECT * FROM dh_tl WHERE name LIKE '_c_';
+----+------+------+
| id | name | goal |
+----+------+------+
|  2 | bcd  |   99 |
+----+------+------+
1 row in set (0.00 sec)

MariaDB [dh]> 

3. ORDER BY　　

• SQL原语：SELECT column_name,column_name FROM table_name ORDER BY column_name,column_name ASC|DESC;默认升序

　　以上面的例子，进行练习：

MariaDB [dh]> SELECT * FROM dh_tl ORDER BY goal;
+----+------+------+
| id | name | goal |
+----+------+------+
|  3 | cdf  |   98 |
|  2 | bcd  |   99 |
|  1 | abc  |  100 |
+----+------+------+
3 rows in set (0.00 sec)

MariaDB [dh]> 

4. UNION

• SELECT column_name(s) FROM table1
  UNION
  SELECT column_name(s) FROM table2;不含重复数据
• SELECT column_name(s) FROM table1
  UNION ALL
  SELECT column_name(s) FROM table2;含有重复数据
• 请注意，UNION 内部的每个 SELECT 语句必须拥有相同数量的列。列也必须拥有相似的数据类型。同时，每个 SELECT 语句中的列的顺序必须相同。

　　创建一个新表，并结合使用上面的例子，练习如下：

//这里说明必须union联合的select语句必须有相同的column数

MariaDB [dh]> SELECT id,goal FROM dh_tl UNION SELECT same_name FROM by_tl ORDER BY id;   
ERROR 1222 (21000): The used SELECT statements have a different number of columns

//虽然输出了结果，但是数据结果毫无意义，所以union联合的select语句应该选择相同的列
MariaDB [dh]> SELECT id,goal FROM dh_tl UNION SELECT id,same_name FROM by_tl ORDER BY id;
+----+------+
| id | goal |
+----+------+
|  1 | 100  |
|  1 | qwe  |
|  2 | 99   |
|  2 | rty  |
|  3 | 98   |
|  3 | uio  |
+----+------+
6 rows in set (0.01 sec)

//虽然id有重复，但是因为goal没有重复，所以还是输出了所有的结果

MariaDB [dh]> SELECT id,goal FROM dh_tl UNION SELECT id,goal FROM by_tl ORDER BY id;         
+----+------+
| id | goal |
+----+------+
|  1 |  100 |
|  1 |   97 |
|  2 |   99 |
|  2 |   96 |
|  3 |   98 |
|  3 |   95 |
+----+------+
6 rows in set (0.00 sec)

//与上面形成对比

MariaDB [dh]> SELECT id FROM dh_tl UNION SELECT id FROM by_tl ORDER BY id;          
+----+
| id |
+----+
|  1 |
|  2 |
|  3 |
+----+
3 rows in set (0.00 sec)

MariaDB [dh]> SELECT id FROM dh_tl UNION ALL SELECT id FROM by_tl ORDER BY id;
+----+
| id |
+----+
|  1 |
|  1 |
|  2 |
|  2 |
|  3 |
|  3 |
+----+
6 rows in set (0.00 sec)

MariaDB [dh]> 

5. JOIN

• JOIN 按照功能大致分为如下三类：
  
  • INNER JOIN（内连接,或等值连接）：获取两个表中字段匹配关系的记录。
  • LEFT JOIN（左连接）：获取左表所有记录，即使右表没有对应匹配的记录。
  • RIGHT JOIN（右连接）： 与 LEFT JOIN 相反，用于获取右表所有记录，即使左表没有对应匹配的记录。

　　实践是理解概念的好方法：

---两个表的id相同的记录。
MariaDB [dh]> SELECT dh_tl.id,dh_tl.name,by_tl.goal FROM dh_tl LEFT JOIN by_tl ON  by_tl.id = dh_tl.id;   
+----+------+------+
| id | name | goal |
+----+------+------+
|  1 | abc  |   97 |
|  2 | bcd  |   96 |
|  3 | cdf  |   95 |
+----+------+------+
3 rows in set (0.00 sec)

---dh_tl插入第四行
MariaDB [dh]> INSERT INTO dh_tl (name,goal)VALUES('bnm',70);
Query OK, 1 row affected (0.02 sec)
---LEFT join
MariaDB [dh]> SELECT dh_tl.id,dh_tl.name,by_tl.goal FROM dh_tl LEFT JOIN by_tl ON  by_tl.id = dh_tl.id;
+----+------+------+
| id | name | goal |
+----+------+------+
|  1 | abc  |   97 |
|  2 | bcd  |   96 |
|  3 | cdf  |   95 |
|  4 | bnm  | NULL |
+----+------+------+
4 rows in set (0.00 sec)

---by_tl插入第四行
MariaDB [dh]> INSERT INTO by_tl (same_name,goal)VALUES('jkl',60);                                         
Query OK, 1 row affected (0.00 sec)

---RIGHT JOIN
MariaDB [dh]> SELECT dh_tl.id,dh_tl.name,by_tl.goal FROM dh_tl RIGHT JOIN by_tl ON  by_tl.id = dh_tl.id;    
+------+------+------+
| id   | name | goal |
+------+------+------+
|    1 | abc  |   97 |
|    2 | bcd  |   96 |
|    3 | cdf  |   95 |
|    4 | bnm  |   60 |
+------+------+------+
4 rows in set (0.00 sec)

---RIGHT JOIN第二个例子
MariaDB [dh]> SELECT dh_tl.id,dh_tl.name,by_tl.goal FROM dh_tl RIGHT JOIN by_tl ON  by_tl.same_name = dh_tl.name;
+------+------+------+
| id   | name | goal |
+------+------+------+
| NULL | NULL |   97 |
| NULL | NULL |   96 |
| NULL | NULL |   95 |
| NULL | NULL |   60 |
+------+------+------+
4 rows in set (0.00 sec)

---JOIN的例子
MariaDB [dh]> SELECT dh_tl.id,dh_tl.name,by_tl.goal FROM dh_tl JOIN by_tl ON  by_tl.same_name = dh_tl.name;      
Empty set (0.00 sec)

MariaDB [dh]> 

　　上述结果可以使用下面三个图理解，图片来自runoob比较简单：

6. 小结

　　索引部分推迟到明天，今天学习了LIKE、JOIN、UNION、ORDER BY的使用，并且接触了SQL脚本的概念，其和shell脚本和shell命令的关系类似，如果有后者的基础，上手也会非常快。