197 lines
3.4 KiB
Markdown
197 lines
3.4 KiB
Markdown
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## MySQL 的一些简单命令
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我们可以在 Navicat Premium 软件中，创建数据库和表，然后输入查询命令来查询数据。选择菜单栏「查询->新建查询->输入 sql 命令->运行」即可，效果如下：
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我们还可以直接在终端输入命令行来操作。
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注意，在 Mac 终端执行 sql 命令时，命令的末尾必须加上`;`（英文格式的分号）。效果如下：
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MySQL 命令行的一些简单命令如下。
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**以 root 身份进入命令行**：
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```
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mysql -u root -p
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```
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**查看有哪些数据库**：
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```sql
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show databases;
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```
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**选择进入指定的数据库**：
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```sql
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use xxx_database;
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# 举例
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use qianguyihao_database;
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```
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**在当前数据库中，查看有哪些表**：
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```sql
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show tables;
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```
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**在当前数据库中，查询指定表的全部数据**：
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```sql
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SELECT * FROM xxx_table;
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# 举例
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SELECT * FROM qianguyihao_student_table
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```
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**删除指定的表**：
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```sql
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drop table xxx;
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# 举例
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drop table qianguyihao_student_table;
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```
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**删除指定的数据库**：
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```sql
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drop database qianguyihao_student_table;
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```
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**创建一个数据库**：
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```sql
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create database qianguyihao_database2;
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```
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## where 条件查询
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使用 `where` 子句可以对表中的数据进行筛选，结果为true的行会出现在查询结果中。
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语法格式如下：
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```sql
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SELECT * FROM 表名 where 条件;
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```
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上面的语法格式中，`条件` 具体要怎么写呢？这个可能有很多种情况。我们继续往下看。
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### 比较运算符
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- `=` 等于
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- `>` 大于
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- `>=` 大于等于
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- `<` 小于
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- `<=` 小于等于
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- `!=`：不等于
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- `age > 20`：查询 age 大于 30 的数据
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**举例**：
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```sql
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# 查询 age 大于 20 的数据
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SELECT * FROM qianguyihao_table WHERE age > 20;
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```
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### 逻辑运算符
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- AND
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- OR
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- NOT
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**举例**：
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```sql
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# 查询 age 在20至30之间的数据
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SELECT * FROM qianguyihao_table WHERE age BETWEEN 20 AND 30;
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```
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### 范围查询
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- `in` 表示在一个非连续的范围内。
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- `between ... and ...` 表示在一个连续的范围内
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举例：
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```sql
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# 查询 name 为 千古壹号 或者 许嵩的数据
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SELECT * FROM qianguyihao_table WHERE name IN ['千古壹号', '许嵩'];
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SELECT * FROM qianguyihao_table WHERE age BETWEEN 20 AND 30;
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```
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### 模糊查询
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- `like`
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- `%` 表示任意多个任意字符
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- `_` 表示一个任意字符
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`%` 符号举例：
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```sql
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# 查询标题中包含“前端”这两个字的数据（“前端”这两个字的前后可能都有内容）
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select * from qianguyihao_table where `title` like "%前端%";
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# 查询标题以“前端”开头的数据
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select * from qianguyihao_table where `title` like "%前端";
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```
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`_`符号举例：
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```sql
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# 查询标题，查询条件是：标题中至少有五个字符，而且，这五个字符中，前两个字符一定是“千古”开头的。
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SELECT * FROM qianguyihao_table WHERE `title` LIKE "千古___";
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```
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### NULL 的判断
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- `is null` 判断为空
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- `is not null` 判断为非空
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注意，`is null` 和**空字符串**`""` 是有区别的。学过js基础之后，你应该知道：空字符串并非null，只不过是里面的值为空而已；空字符串也是会占有内存空间的。
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举例：
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```sql
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select * from qianguyihao_table where name is not NULL;
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```
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### 优先级
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