<p></p> <p>SELECT statement is the basis for MySQL querying data, and it is crucial to master its basic usage. 1. When querying the entire table data, you can use SELECT * FROM table names, but it is recommended to list specific fields to improve performance and readability; 2. Use the WHERE clause to filter data by condition, supporting comparison and logical operator combination conditions; 3. Sort the results through ORDER BY, default ascending order, and DESC can be specified in descending order; 4. Use LIMIT to control the number of rows, and combine OFFSET to implement paging queries. These basic operations lay a solid foundation for more complex queries. </p> <p><img src="/static/imghw/default1.png" data-src="https://img.php.cn/upload/article/000/000/000/175216999365327.jpg" class="lazy" alt="Writing Basic SELECT Queries to Retrieve Data in MySQL"></p> <p> Querying data is one of the core operations of using MySQL, and the <code>SELECT</code> statement is the basis for implementing this function. Mastering the basic <code>SELECT</code> query writing can not only help you quickly obtain the information you need, but also lay the foundation for more complex queries. </p> <img src="/static/imghw/default1.png" data-src="https://img.php.cn/upload/article/000/000/000/175216999482179.jpeg" class="lazy" alt="Writing Basic SELECT Queries to Retrieve Data in MySQL"><hr> <h3 id="Query-the-data-of-the-entire-table"> Query the data of the entire table</h3> <p> If you want to know what data is in a table, the most direct way is to use <code>SELECT *</code> to query all columns. </p> <img src="/static/imghw/default1.png" data-src="https://img.php.cn/upload/article/000/000/000/175216999561467.jpeg" class="lazy" alt="Writing Basic SELECT Queries to Retrieve Data in MySQL"><pre class='brush:php;toolbar:false;'> SELECT * FROM users;</pre><p> <code>*</code> here means "all columns", and <code>users</code> are the table name. This statement returns all rows and all fields in the <code>users</code> table.<br /> However, it should be noted that if the table has a large amount of data, this writing may cause performance problems or return too much unnecessary information. Therefore, in actual development, if all fields are not particularly needed, it is recommended to clearly list the columns to be queried, such as:</p><pre class='brush:php;toolbar:false;'> SELECT id, name, email FROM users;</pre><p> This reduces the burden on the database and makes it easier to read the results. </p><img src="/static/imghw/default1.png" data-src="https://img.php.cn/upload/article/000/000/000/175216999697344.jpeg" class="lazy" alt="Writing Basic SELECT Queries to Retrieve Data in MySQL" /><hr /><h3 id="Filter-data-by-conditional"> Filter data by conditional</h3><p> Many times we don’t want to find out all the data, but just want to see records that meet certain conditions. At this time, the <code>WHERE</code> clause must be used.</p><pre class='brush:php;toolbar:false;'> SELECT id, name, email FROM users WHERE status = &#39;active&#39;;</pre><p> In the example above, we only check users whose status is <code>active</code> . <code>WHERE</code> can be followed by various comparison operators, such as:</p><ul><li> <code>=</code> equal to</li><li> <code><></code> or <code>!=</code> is not equal to</li><li> <code>></code> Big than</li><li> <code><</code> Lower than</li><li> <code>>=</code> Greater than or equal to</li><li> <code><=</code> Less than or equal to</li></ul><p> Multiple conditions can also be combined with logical operators:</p><ul><li> <code>AND</code></li><li> <code>OR</code></li><li> <code>NOT</code></li></ul><p> For example:</p><pre class='brush:php;toolbar:false;'> SELECT * FROM orders WHERE amount > 100 AND status = &#39;paid&#39;;</pre><p> This selects orders with an amount greater than 100 and a paid status.</p><hr /><h3 id="Sort-the-query-results"> Sort the query results</h3><p> By default, the order of query results is uncertain. If you want to display by sorting a field, you can use <code>ORDER BY</code> .</p><pre class='brush:php;toolbar:false;'> SELECT name, salary FROM employees ORDER BY salary DESC;</pre><p> The above statement will arrange employee information from high to low according to salary. <code>DESC</code> means descending order, if not written, the default is ascending order ( <code>ASC</code> ).</p><p> You can also sort it according to multiple fields, such as first by department and then by salary:</p><pre class='brush:php;toolbar:false;'> SELECT department, name, salary FROM employees ORDER BY department ASC, salary DESC;</pre><p> This way, it can be displayed within each department according to the salary level.</p><hr /><h3 id="Control-the-number-of-rows-returned"> Control the number of rows returned</h3><p> Sometimes you only need the first few pieces of data, not the full result. <code>LIMIT</code> can be used to limit the number of rows returned.</p><pre class='brush:php;toolbar:false;'> SELECT * FROM products ORDER BY price DESC LIMIT 5;</pre><p> This statement will find the five highest priced items. If you want to view pages, you can also add an offset:</p><pre class='brush:php;toolbar:false;'> SELECT * FROM products ORDER BY price DESC LIMIT 5 OFFSET 10;</pre><p> It means skipping the first 10 pieces and taking the next 5 pieces of data.</p> <hr> <p> Basically that's all. Although they are basic operations, they form the core part of most data queries. After mastering it, you can start trying more advanced usages such as joining tables and aggregate functions.</p>

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