SQL知识点详细总结（附操作图解）
Posted 谁曾见过风-
tags:
篇首语：本文由小常识网(cha138.com)小编为大家整理，主要介绍了SQL知识点详细总结（附操作图解）相关的知识，希望对你有一定的参考价值。
目录
查询已有数据库和表
1.打开CMD并登录mysql:mysql -uroot -p
C:\\Windows\\system32>mysql -uroot -p
Enter password: ******
Welcome to the MySQL monitor. Commands end with ; or \\g.
Your MySQL connection id is 8
Server version: 8.0.29 MySQL Community Server - GPL
Copyright (c) 2000, 2022, Oracle and/or its affiliates.
Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.
Type 'help;' or '\\h' for help. Type '\\c' to clear the current input statement.2.查询已有数据库：SHOW DATABASES;
mysql> show databases;
+--------------------+
| Database |
+--------------------+
| class |
| information_schema |
| mysql |
| performance_schema |
| product |
| shop |
| sys |
+--------------------+
7 rows in set (0.02 sec)3.查询指定数据库中已有的表:SHOW TABLES;
mysql> show tables;
+-----------------+
| Tables_in_class |
+-----------------+
| student |
+-----------------+
1 row in set (0.00 sec)4.查询表的结构：DESC 表名称;
mysql> desc student;
+--------------+--------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+--------------+--------------+------+-----+---------+-------+
| student_id | char(4) | NO | PRI | NULL | |
| student_name | varchar(100) | NO | | NULL | |
| test_scores | int | NO | | NULL | |
| student_sex | varchar(100) | YES | | NULL | |
| birth_date | date | YES | | NULL | |
+--------------+--------------+------+-----+---------+-------+
5 rows in set (0.04 sec)SELECT语句+列名称 查询列
1.查询表中所有的列:SELECT * FROM 列名称;
mysql> select * from student;
+------------+--------------+-------------+-------------+------------+
| student_id | student_name | test_scores | student_sex | birth_date |
+------------+--------------+-------------+-------------+------------+
| 01 | 张三 | 90 | 男 | 2002-02-23 |
| 02 | 李梅 | 95 | 女 | 2001-11-12 |
| 03 | 王二 | 89 | 男 | 2002-05-01 |
| 04 | 钱进 | 80 | 男 | 2002-11-12 |
| 05 | 韩雪 | 90 | 女 | 2001-12-25 |
+------------+--------------+-------------+-------------+------------+
5 rows in set (0.00 sec)2.查询表中需要的列:SELECT 列名称1，列名称2，列名称3 FROM 表名称;
mysql> select student_id,student_sex,student_name from student;
+------------+-------------+--------------+
| student_id | student_sex | student_name |
+------------+-------------+--------------+
| 01 | 男 | 张三 |
| 02 | 女 | 李梅 |
| 03 | 男 | 王二 |
| 04 | 男 | 钱进 |
| 05 | 女 | 韩雪 |
+------------+-------------+--------------+
5 rows in set (0.00 sec)SELECT语句+AS关键字 设定别名
1.SELECT 列名称1 AS 别名1,列名称2 AS 别名2 FROM 表名称;
mysql> select student_id as id,student_sex as sex,student_name as name from student;
+----+------+------+
| id | sex | name |
+----+------+------+
| 01 | 男 | 张三 |
| 02 | 女 | 李梅 |
| 03 | 男 | 王二 |
| 04 | 男 | 钱进 |
| 05 | 女 | 韩雪 |
+----+------+------+
5 rows in set (0.00 sec)2.设定中文别名——需要用双引号“”标识
SELECT 列名称1 AS "中文别名1",列名称2 AS "中文别名2" FROM 表名称;
mysql> select student_id as "学号",student_sex as "性别",student_name as "姓名" from student;
+------+------+------+
| 学号 | 性别 | 姓名 |
+------+------+------+
| 01 | 男 | 张三 |
| 02 | 女 | 李梅 |
| 03 | 男 | 王二 |
| 04 | 男 | 钱进 |
| 05 | 女 | 韩雪 |
+------+------+------+
5 rows in set (0.00 sec)SELECT语句+常数
查询常数：SELECT '字符串常数' AS string,数字常数 AS number,'日期常数' AS date FROM 表名称;
mysql> select '学生' AS string,90 AS number,'2002-02-23' AS date,student_id,student_name from student;
+--------+--------+------------+------------+--------------+
| string | number | date | student_id | student_name |
+--------+--------+------------+------------+--------------+
| 学生 | 90 | 2002-02-23 | 01 | 张三 |
| 学生 | 90 | 2002-02-23 | 02 | 李梅 |
| 学生 | 90 | 2002-02-23 | 03 | 王二 |
| 学生 | 90 | 2002-02-23 | 04 | 钱进 |
| 学生 | 90 | 2002-02-23 | 05 | 韩雪 |
+--------+--------+------------+------------+--------------+
5 rows in set (0.00 sec)SELECT语句+DISTINCT 删除重复行
1.单列数据合并：SELECT DISTINCT 列名称 FROM 表名称;
注：使用DISTINCT时，NULL也被视为一类数据，也会被合并。
mysql> select distinct test_scores from student;
+-------------+
| test_scores |
+-------------+
| 90 |
| 95 |
| 89 |
| 80 |
+-------------+
4 rows in set (0.01 sec)2.多列数据合并：SELECT DISTINCT 列名称1,列名称2,列名称3 FROM 表名称;
SELECT语句+LIMIT 限制返回行数
1.选取前N行：SELECT 列名称 FROM 表名称 LIMIT （行数）;
2.选取中间行（m-n）：
SELECT 列名称 FROM 表名称 LIMIT n-m+1（行数） OFFSET m-1（第一行偏移量）;
或者SELECT 列名称 FROM 表名称 LIMIT m-1(第一行偏移量）,n-m+1（返回的最大行数）;
注：Limit M,N——跳过M条记录向后选取N条数据。
3.统计表格共有几行数据：SELECT COUNT(*) FROM 表名称;
select student_name from student limit 2;
+--------------+
| student_name |
+--------------+
| 张三 |
| 李梅 |
+--------------+mysql> select student_id from student limit 1,4;
+------------+
| student_id |
+------------+
| 02 |
| 03 |
| 04 |
| 05 |
+------------+
4 rows in set (0.00 sec)mysql> select count(*) from student;
+----------+
| count(*) |
+----------+
| 5 |
+----------+
1 row in set (0.01 sec)SELECT语句+WHERE 条件选取
1.SELECT 列名称 FROM 表名称 WHERE 条件表达式;
mysql> select student_name,test_scores from student where test_scores=90;
+--------------+-------------+
| student_name | test_scores |
+--------------+-------------+
| 张三 | 90 |
| 韩雪 | 90 |
+--------------+-------------+
2 rows in set (0.00 sec)mysql> select student_name from student where test_scores=90;
+--------------+
| student_name |
+--------------+
| 张三 |
| 韩雪 |
+--------------+
2 rows in set (0.00 sec)2.SELECT+WHERE+BETWEEN+AND：
SELECT 列名称 FROM 表名称 WHERE 列名称 BETWEEN 条件1 AND 条件2;
mysql> select student_name,test_scores from student where test_scores between 80 and 90;
+--------------+-------------+
| student_name | test_scores |
+--------------+-------------+
| 张三 | 90 |
| 王二 | 89 |
| 钱进 | 80 |
| 韩雪 | 90 |
+--------------+-------------+
4 rows in set (0.00 sec)SELECT语句+算数运算符
1.算数运算符：+、-、*、/（加、减、乘、除）
2.（）——括号内的运算表达式优先运算
3.所有包含NULL的计算，结果肯定是NULL
mysql> select student_name,test_scores,test_scores/2 FROM student;
+--------------+-------------+---------------+
| student_name | test_scores | test_scores/2 |
+--------------+-------------+---------------+
| 张三 | 90 | 45.0000 |
| 李梅 | 95 | 47.5000 |
| 王二 | 89 | 44.5000 |
| 钱进 | 80 | 40.0000 |
| 韩雪 | 90 | 45.0000 |
+--------------+-------------+---------------+
5 rows in set (0.00 sec)mysql> select student_name,test_scores,(test_scores+10)*2 from student;
+--------------+-------------+--------------------+
| student_name | test_scores | (test_scores+10)*2 |
+--------------+-------------+--------------------+
| 张三 | 90 | 200 |
| 李梅 | 95 | 210 |
| 王二 | 89 | 198 |
| 钱进 | 80 | 180 |
| 韩雪 | 90 | 200 |
+--------------+-------------+--------------------+
5 rows in set (0.00 sec)mysql> select student_name,test_scores,test_scores+NULL from student;
+--------------+-------------+------------------+
| student_name | test_scores | test_scores+NULL |
+--------------+-------------+------------------+
| 张三 | 90 | NULL |
| 李梅 | 95 | NULL |
| 王二 | 89 | NULL |
| 钱进 | 80 | NULL |
| 韩雪 | 90 | NULL |
+--------------+-------------+------------------+
5 rows in set (0.00 sec)mysql> select (1000+2000)*2 as caculation;
+------------+
| caculation |
+------------+
| 6000 |
+------------+
1 row in set (0.00 sec)SELECT语句+比较运算符
| 等于 | = |
| 不等于 | <> |
| 大于等于 | >= |
| 大于 | > |
| 小于等于 | <= |
| 小于 | < |
注：1.比较运算符位置不能颠倒。
2.字符串类型的数据原则上按照字典顺序进行排序，不能与数字的大小顺序混淆。
3.不能对NULL使用比较运算符，选取NULL或者不是NULL记录条件表达式使用IS NULL或者IS NOT NULL。
mysql> select student_name,test_scores
-> from student
-> where test_scores<>90;
+--------------+-------------+
| student_name | test_scores |
+--------------+-------------+
| 李梅 | 95 |
| 王二 | 89 |
| 钱进 | 80 |
+--------------+-------------+
3 rows in set (0.00 sec)mysql> select student_name,student_sex,birth_date from student
-> where birth_date>='2002-01-01';
+--------------+-------------+------------+
| student_name | student_sex | birth_date |
+--------------+-------------+------------+
| 张三 | 男 | 2002-02-23 |
| 王二 | 男 | 2002-05-01 |
| 钱进 | 男 | 2002-11-12 |
+--------------+-------------+------------+
3 rows in set (0.00 sec)mysql> select student_name,student_id,test_scores from student
-> where test_scores-60>=30;
+--------------+------------+-------------+
| student_name | student_id | test_scores |
+--------------+------------+-------------+
| 张三 | 01 | 90 |
| 李梅 | 02 | 95 |
| 韩雪 | 05 | 90 |
+--------------+------------+-------------+
3 rows in set (0.00 sec)mysql> -- 创建表
mysql> CREATE TABLE string(number CHAR(4) NOT NULL,PRIMARY KEY(number));
Query OK, 0 rows affected (0.05 sec)
mysql> -- 插入数据
mysql> START TRANSACTION;
Query OK, 0 rows affected (0.00 sec)
mysql> insert into string(number) values('1'),('2'),('3'),('4'),('11'),('12'),('21'),('33'),('123');
Query OK, 9 rows affected (0.00 sec)
Records: 9 Duplicates: 0 Warnings: 0
mysql> -- 查询所有列
mysql> select * from string;
+--------+
| number |
+--------+
| 1 |
| 11 |
| 12 |
| 123 |
| 2 |
| 21 |
| 3 |
| 33 |
| 4 |
+--------+
9 rows in set (0.00 sec)
mysql> commit;
Query OK, 0 rows affected (0.04 sec)
mysql> -- 选出大于'21'的数据
mysql> select number from string where number>'21';
+--------+
| number |
+--------+
| 3 |
| 33 |
| 4 |
+--------+
3 rows in set (0.04 sec)SELECT语句+逻辑运算符
1.逻辑运算符：NOT、AND、OR
2.真值——值为真（TRUE）或者假（FALSE）其中之一的值。
3.逻辑运算符把比较运算符返回的真值等进行操作：
AND运算符两侧真值均为真则反馈真，否则为假；
OR运算符两侧真值均为假则返回假，否则为真；
NOT转换真为假，假为真；
4.只有SQL中的逻辑运算被称为三值逻辑，第三种值为不确定（UNKNOWN），需要用到IS NULL或者IS NOT NULL。
5.AND运算符的优先级高于OR运算符，要优先执行OR运算符需要使用括号。
NOT运算符的运用：SELECT 列名称 FROM 表名称 WHERE NOT 条件表达式;
mysql> select student_name,student_sex,test_scores from student where NOT test_scores<90;
+--------------+-------------+-------------+
| student_name | student_sex | test_scores |
+--------------+-------------+-------------+
| 张三 | 男 | 90 |
| 李梅 | 女 | 95 |
| 韩雪 | 女 | 90 |
+--------------+-------------+-------------+
3 rows in set (0.00 sec)AND运算符的运用：SELECT 列名称 FROM 表名称 WHERE 条件1 AND 条件2;
mysql> select student_name,student_sex,test_scores from student where test_scores<=90 and student_sex='男';
+--------------+-------------+-------------+
| student_name | student_sex | test_scores |
+--------------+-------------+-------------+
| 张三 | 男 | 90 |
| 王二 | 男 | 89 |
| 钱进 | 男 | 80 |
+--------------+-------------+-------------+
3 rows in set (0.00 sec)OR运算符的运用：SELECT 列名称 FROM 表名称 WHERE 条件1 OR 条件2;
mysql> select student_name,student_sex,test_scores from student where test_scores<85 OR student_sex='女';
+--------------+-------------+-------------+
| student_name | student_sex | test_scores |
+--------------+-------------+-------------+
| 李梅 | 女 | 95 |
| 钱进 | 男 | 80 |
| 韩雪 | 女 | 90 |
+--------------+-------------+-------------+
3 rows in set (0.00 sec)注释的书写方法
1.注释——SQL语句中用来标识说明或者注意事项，对SQL的执行没有任何影响。
2.单行注释——写在同一行，写在-- 之后（需要在--后面加半角空格）
3.多行注释——可以跨行，写在/*和*/之间
未完待续。。。。。。
以上是关于SQL知识点详细总结（附操作图解）的主要内容，如果未能解决你的问题，请参考以下文章
内连接，外链接（左连接右连接全连接），交叉连接大总结+附SQL JOINS图解[转]