[Teach a fish to fish] How to perform a simple SELECT query? How to filter using WHERE clause?

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How to perform a simple SELECT query? How to filter using WHERE clause?

Executing a simple SELECT query and filtering using the WHERE clause are basic operations for database queries using SQL (Structured Query Language).

The steps to perform a simple SELECT query are as follows:

1. Write SQL query statements: Use the SELECT statement to specify the columns and tables to be queried.

SELECT column1, column2, ...
FROM table_name;

1. Execute query: Send the query statement to the database system for execution.

2. Get query results: Receive query results from the database system, usually returned in tabular form.

When filtering using the WHERE clause, query results can be filtered based on specific conditions. The WHERE clause follows the SELECT statement as follows:

SELECT column1, column2, ...
FROM table_name
WHERE condition;

Among them, conditionis a logical expression used to specify filter conditions. Only rows that meet the criteria will be included in the query results.

Detailed explanation of the principle:

The principle of executing a simple SELECT query and filtering using the WHERE clause is as follows:

1. Query parsing: After the database system receives the query statement, it first performs syntax analysis to ensure the correctness of the query statement.

2. Query optimization: The database system will optimize query statements to improve query performance. The optimization process includes selecting appropriate indexes, determining query execution plans, etc.

3. Query execution: According to the optimized execution plan, the database system begins to execute the query operation. It retrieves data from the storage engine based on the tables and conditions specified in the query statement.

4. Data filtering: When executing a query, the conditions in the WHERE clause are applied to the retrieved data rows. Only rows that meet the conditions will be included in the query results, other rows will be filtered out.

5. Query result return: The database system returns data rows that meet the conditions as query results to the user or application.

Database query underlying architecture flow chart:

The underlying architecture flow chart of database query is as follows:

+---------------------+
|    Client Application    |
+---------------------+
            |
            |
            v
+---------------------+
|   Database Driver   |
|   (Connection,       |
|   Query Execution)   |
+---------------------+
            |
            |
            v
+---------------------+
|   Database Server   |
| (Query Parsing,      |
|   Optimization,       |
|   Execution)           |
+---------------------+
            |
            |
            v
+---------------------+
|   Storage Engine   |
|    (Data Storage    |
|    and Retrieval)     |
+---------------------+

In the underlying architecture of database query, the client application establishes a connection with the database server through the database driver. The driver is responsible for handling connections, executing SQL commands and receiving results. After receiving the SQL command, the database server performs query optimization, execution plan and actual data operations. Data storage and retrieval are handled by the storage engine, which manages the physical storage and retrieval of data.

Usage scenario explanation:

Executing a simple SELECT query and filtering using the WHERE clause is suitable for the following scenarios:

1. Data retrieval: When you need to retrieve data for a specific column or all columns from the database, you can use SELECT query.

2. Data filtering: Use the WHERE clause to filter data according to specific conditions and only return data rows that meet the conditions.

3. Data statistics: SELECT queries can be used to aggregate data, such as calculating average values, sums, maximum values, minimum values, etc.

4. Data sorting: Use SELECT queries to sort data based on specified columns to obtain results in a specific order.

Code example implementation:

Here is a simple example showing how to perform a SELECT query and filter using the WHERE clause:

-- 查询所有用户的姓名和年龄
SELECT name, age
FROM users;

Here are some common database products and query engines for your reference:

Database products:

1. MySQL: A popular open source relational database management system.

   • Official website: https://www.mysql.com/ ↗

2. PostgreSQL: A powerful open source object-relational database management system.

   • Official website: https://www.postgresql.org/ ↗

3. Oracle Database: A commercial relational database management system widely used in enterprise applications.

   • Official website: https://www.oracle.com/database/ ↗

4. Microsoft SQL Server: A relational database management system provided by Microsoft.

   • Official website: https://www.microsoft.com/en-us/sql-server/ ↗

5. MongoDB: A popular open source document database that uses NoSQL data storage.

   • Official website: https://www.mongodb.com/ ↗

Query engine:

1. Apache Hive: Hadoop-based data warehouse infrastructure for querying and analyzing large-scale data sets.

   • Official website: https://hive.apache.org/ ↗

2. Apache Cassandra: A highly scalable distributed NoSQL database for processing large-scale data sets.

   • Official website: https://cassandra.apache.org/ ↗

3. Elasticsearch: A distributed search and analytics engine for searching, analyzing, and visualizing large amounts of data in real time.

   • Official website: https://www.elastic.co/elasticsearch/ ↗