Common SQL statements include: 1. CREATE TABLE creates tables, such as CREATE TABLE employees (id INT PRIMARY KEY, name VARCHAR(100), salary DECIMAL(10, 2)); 2. CREATE INDEX creates indexes, such as CREATE INDEX idx_name ON employees(name); 3. INSERT INTO inserts data, such as INSERT INTO employees (id, name, salary) VALUES (1, 'John Doe', 75000.00); 4. SELECT query data, such as SELECT FROM employees; 5. UPDATE update data, such as UPDATE employees SET salary = salary 1.10; 6. DELETE delete data, such as DELETE FROM employees WHERE salary 75000;9. EXPLAIN analyze query performance, such as EXPLAIN SELECT FROM employees WHERE salary > 75000;10. Use parameterized queries to prevent SQL injection, such as using cursor.execute("SELECT FROM employees in Python WHERE name = ?", (user_input,)). These statements cover the main operations and security measures of SQL, and mastering these can greatly improve database management efficiency.

When processing SQL databases, mastering commonly used SQL statements is the basic skill of every developer and database administrator. Today, we will discuss the collection of common statements for SQL databases, hoping to help you better understand and use SQL.

SQL, a structured query language, is a standard language used to manage and operate relational databases. Whether you are a beginner or a developer with some experience, proficiency in using SQL statements can greatly improve your work efficiency.

Let's start with some of the most basic SQL statements.

For the operation of creating tables, SQL's CREATE TABLE statement is essential. Suppose we want to create a table called employees , including id , name and salary fields, we can write it like this:

 CREATE TABLE employees (
    id INT PRIMARY KEY,
    name VARCHAR(100),
    Salary DECIMAL(10, 2)
);

This statement not only defines the structure of the table, but also sets id as the primary key. In actual operation, you may encounter situations where you need to create an index. You can use the CREATE INDEX statement to improve query performance:

 CREATE INDEX idx_name ON employees(name);

Now, suppose we need to insert some data into the employees table, we can use the INSERT INTO statement:

 INSERT INTO employees (id, name, salary) VALUES (1, 'John Doe', 75000.00);
INSERT INTO employees (id, name, salary) VALUES (2, 'Jane Smith', 80000.00);

These statements show how to insert single rows of data. If you need to insert data in batches, you can use a multi-line INSERT INTO statement, or consider using the LOAD DATA INFILE statement to import data from the file.

Querying data is one of the core functions of SQL, and SELECT statements are one of our most commonly used tools. For example, to query information for all employees:

 SELECT * FROM employees;

If you just want to query specific fields, you can do this:

 SELECT name, salary FROM employees;

To filter data more accurately, we can use the WHERE clause. For example, check employees with salary above 75,000:

 SELECT * FROM employees WHERE salary > 75000;

In practical applications, you may need to sort the data, and the ORDER BY clause comes in handy:

 SELECT * FROM employees ORDER BY salary DESC;

This query returns the results in order of salary from high to low.

When updating data, the UPDATE statement is our choice. For example, suppose we need to give all employees a 10% salary increase:

 UPDATE employees SET salary = salary * 1.10;

If you only want to update data for specific employees, you can combine the WHERE clause:

 UPDATE employees SET salary = salary * 1.10 WHERE id = 1;

When deleting data, the DELETE statement is our tool. For example, to delete all employee records with salary below 70,000:

 DELETE FROM employees WHERE salary < 70000;

In practice, DELETE statements need to be used with caution, as they permanently delete data. It is recommended to use the SELECT statement to verify the data to be deleted before performing the deletion operation.

In addition to these basic operations, SQL also provides many advanced features, such as JOIN operations, which can be used for associated queries between tables. For example, suppose we have another table departments , we can use JOIN to query information about employees and their departments:

 SELECT employees.name, departments.dept_name
FROM employees
JOIN departments ON employees.dept_id = departments.id;

When using JOIN, you need to pay attention to the differences between different types of JOINs (such as INNER JOIN, LEFT JOIN, RIGHT JOIN, etc.), which will affect the integrity of the query results.

For data aggregation operations, SQL provides GROUP BY and HAVING clauses. For example, to count the average salary for each department:

 SELECT departments.dept_name, AVG(employees.salary) as avg_salary
FROM employees
JOIN departments ON employees.dept_id = departments.id
GROUP BY departments.dept_name
HAVING avg_salary > 75000;

This query not only shows how to use GROUP BY for grouping, but also shows how to use the HAVING clause to filter the results after grouping.

In practical applications, SQL performance optimization is an important topic. The use of indexes, query optimization, and avoiding unnecessary subqueries are all key points to improving SQL query performance. For example, using the EXPLAIN statement can help us analyze the execution plan of a query:

 EXPLAIN SELECT * FROM employees WHERE salary > 75000;

This statement will return the query execution plan and help us understand the performance bottlenecks of the query.

Finally, SQL security issues, especially SQL injection attacks, are something that every developer needs to pay attention to. Using parameterized queries and precompiled statements is an effective way to prevent SQL injection. For example, when using SQLite3 library in Python:

 import sqlite3

conn = sqlite3.connect('example.db')
cursor = conn.cursor()

# Use parameterized query to prevent SQL injection of user_input = "John Doe"
cursor.execute("SELECT * FROM employees WHERE name = ?", (user_input,))

for row in cursor.fetchall():
    print(row)

conn.close()

This example shows how to use parameterized queries to prevent SQL injection attacks and ensure the security of the database.

Through the introduction and examples of these commonly used SQL statements, I hope you can better understand and use SQL and be more handy in actual work. SQL is a huge and complex field, and continuous learning and practice are the key to mastering it.

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