SQL Classic 50 Question Answers

SQL (Structured Query Language) is a programming language used to create, manage, and query databases. The main functions include: creating databases and tables, inserting, updating and deleting data, sorting and filtering results, aggregating functions, joining tables, subqueries, operators, functions, keywords, data manipulation/definition/control language, connection types, query optimization, security, tools, resources, versions, common errors, debugging techniques, best practices, trends and row locking.

SQL Classic 50 Question Answers

Q: What is SQL used for?

Answer: SQL (Structured Query Language) is a programming language used to interact with a database, allowing users to create, manage, and query data.

1. Create a database

 <code class="sql">CREATE DATABASE my_database;</code>

2. Create a table

 <code class="sql">CREATE TABLE my_table ( id INTEGER PRIMARY KEY, name TEXT, age INTEGER );</code>

3. Insert data

 <code class="sql">INSERT INTO my_table (id, name, age) VALUES (1, 'John', 30);</code>

4. Query data

 <code class="sql">SELECT * FROM my_table;</code>

5. Update data

 <code class="sql">UPDATE my_table SET name = 'John Doe' WHERE id = 1;</code>

6. Delete data

 <code class="sql">DELETE FROM my_table WHERE id = 1;</code>

7. Sort results

 <code class="sql">SELECT * FROM my_table ORDER BY name;</code>

8. Filter results

 <code class="sql">SELECT * FROM my_table WHERE age > 30;</code>

9. Grouping results

 <code class="sql">SELECT age, COUNT(*) AS count FROM my_table GROUP BY age;</code>

10. Aggregation Function

 <code class="sql">SELECT SUM(age) FROM my_table;</code>

11. Connection table

 <code class="sql">SELECT * FROM customers c JOIN orders o ON c.id = o.customer_id;</code>

12. Subquery

 <code class="sql">SELECT * FROM my_table WHERE id IN (SELECT id FROM my_second_table);</code>

13. EXISTS keywords

 <code class="sql">SELECT * FROM my_table WHERE EXISTS (SELECT 1 FROM my_second_table WHERE id = my_table.id);</code>

14. NOT EXISTS keywords

 <code class="sql">SELECT * FROM my_table WHERE NOT EXISTS (SELECT 1 FROM my_second_table WHERE id = my_table.id);</code>

15. UNION operator

 <code class="sql">SELECT * FROM my_table UNION SELECT * FROM my_second_table;</code>

16. INTERSECT operator

 <code class="sql">SELECT * FROM my_table INTERSECT SELECT * FROM my_second_table;</code>

17. EXCEPT operator

 <code class="sql">SELECT * FROM my_table EXCEPT SELECT * FROM my_second_table;</code>

18. LIKE operator

 <code class="sql">SELECT * FROM my_table WHERE name LIKE '%John%';</code>

19. BETWEEN operator

 <code class="sql">SELECT * FROM my_table WHERE age BETWEEN 20 AND 30;</code>

20. NULL value

 <code class="sql">SELECT * FROM my_table WHERE age IS NULL;</code>

21. Foreign Key Constraints

 <code class="sql">ALTER TABLE my_table ADD FOREIGN KEY (customer_id) REFERENCES customers (id);</code>

22. Unique constraints

 <code class="sql">ALTER TABLE my_table ADD UNIQUE INDEX (name);</code>

23. Primary key constraints

 <code class="sql">ALTER TABLE my_table ADD PRIMARY KEY (id);</code>

24. Transactions

 <code class="sql">BEGIN TRANSACTION; -- 执行一系列查询COMMIT;</code>

25. Stored procedures

 <code class="sql">CREATE PROCEDURE my_procedure (IN param1 INT) AS BEGIN -- 存储过程代码END;</code>

26. Function

 <code class="sql">CREATE FUNCTION my_function (param1 INT) RETURNS INT AS BEGIN -- 函数代码END;</code>

27. View

 <code class="sql">CREATE VIEW my_view AS SELECT * FROM my_table;</code>

28. Cursor

 <code class="sql">DECLARE cursor_name CURSOR FOR SELECT * FROM my_table;</code>

29. Trigger

 <code class="sql">CREATE TRIGGER my_trigger ON my_table FOR DELETE AS BEGIN -- 触发器代码END;</code>

30. Data Type

 <code class="sql">INT (整型) TEXT (文本) FLOAT (浮点数) DATE (日期) TIME (时间) DATETIME (日期时间)</code>

31. Operators

 (加法) - (减法) * (乘法) / (除法) % (模运算)

32. Function

 <code class="sql">SUM() (求和) AVG() (求平均值) MIN() (求最小值) MAX() (求最大值)</code>

33. Keywords

 <code class="sql">SELECT (选择) FROM (从) WHERE (条件) ORDER BY (排序) LIMIT (限制)</code>

34. Data Manipulation Language (DML)

 <code class="sql">INSERT (插入) UPDATE (更新) DELETE (删除)</code>

35. Data Definition Language (DDL)

 <code class="sql">CREATE (创建) DROP (删除) ALTER (修改)</code>

36. Data Control Language (DCL)

 <code class="sql">GRANT (授予) REVOKE (撤销)</code>

37. Connect

 <code class="sql">INNER JOIN (内部连接) LEFT JOIN (左连接) RIGHT JOIN (右连接) FULL JOIN (全连接)</code>

38. Optimize query

使用索引避免嵌套查询使用LIMIT 子句

39. Security

使用强密码启用身份验证使用加密

40. Tools

 <code class="sql">MySQL Workbench phpMyAdmin Navicat</code>

41. Resources

 <code class="sql">MySQL 文档Stack Overflow W3Schools</code>

42. Version

 <code class="sql">MySQL 8.0 MySQL 5.7 MariaDB</code>

43. Common Errors

语法错误表不存在列不存在权限不足

44. Debugging skills

使用EXPLAIN 命令检查错误日志使用调试器

45. Best Practices

使用规范化的模式编写高效的查询备份你的数据库保持你的数据库更新

46. ​​Trends

云数据库NoSQL 数据库机器学习

47. Row lock

 <code class="sql">SELECT ... FOR UPDATE;</code>

48. Partition

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