<p>SQL SELECT statement is used to retrieve data from the database. The basic syntax is SELECT column FROM table; WHERE filtering conditions can be used, such as SELECT name FROM users WHERE age>30; ORDER BY sorting and DISTINCT deduplication are supported; aggregation functions such as COUNT, AVG and GROUP BY implement group statistics, such as SELECT city, COUNT(*) GROUP BY city; the overall structure is flexible and efficient, and is suitable for various query needs. </p> <p><img src="/static/imghw/default1.png" data-src="https://img.php.cn/upload/article/000/000/000/175363577368229.jpeg" class="lazy" alt="How to use the SELECT statement in SQL?"></p> <p> In SQL, <code>SELECT</code> statements are one of the most commonly used operations, mainly used to retrieve data from a database. It can not only query a single field, but also combine multiple tables, filter specific conditions, and even calculate and sort. </p> <img src="/static/imghw/default1.png" data-src="https://img.php.cn/upload/article/000/000/000/175363577368229.jpeg" class="lazy" alt="How to use the SELECT statement in SQL?"><h3 id="Basic-syntax-structure"> Basic syntax structure</h3> <p> The simplest <code>SELECT</code> statement consists of the following parts:</p><pre class='brush:php;toolbar:false;'> SELECT column1, column2, ... FROM table_name;</pre><p> If you are not sure which fields are available, you can also use <code>SELECT *</code> to get all columns of the entire table. However, it is not recommended to use frequently on big data tables, as it will affect performance. </p><img src="/static/imghw/default1.png" data-src="https://img.php.cn/upload/article/000/000/000/175363577529037.jpeg" class="lazy" alt="How to use the SELECT statement in SQL?" /><p> For example, suppose you have a table called <code>users</code> , which has three fields: <code>id</code> , <code>name</code> , and <code>email</code> . If you want to check the names and emails of all users, you can write it like this:</p><pre class='brush:php;toolbar:false;'> SELECT name, email FROM users;</pre><p> This method is clear and efficient, suitable for most basic query needs. </p><img src="/static/imghw/default1.png" data-src="https://img.php.cn/upload/article/000/000/000/175363577738850.jpeg" class="lazy" alt="How to use the SELECT statement in SQL?" /><h3 id="Use-WHERE-to-filter-data"> Use WHERE to filter data</h3><p> Many times we don’t need to find out all the data, but filter the results based on certain conditions. At this time, the <code>WHERE</code> clause must be used.</p><pre class='brush:php;toolbar:false;'> SELECT column1, column2 FROM table_name WHERE condition;</pre><p> For example, just check users whose name is "John":</p><pre class='brush:php;toolbar:false;'> SELECT name, email FROM users WHERE name = &#39;John&#39;;</pre><p> Common conditions also include greater than ( <code>></code> ), less than ( <code><</code> ), not equal to ( <code><></code> or <code>!=</code> ). Multiple conditions can also be combined with <code>AND</code> and <code>OR</code> .</p><ul><li><p> Check users who are older than 30 and whose city is in Beijing:</p><pre class='brush:php;toolbar:false;'> SELECT * FROM users WHERE age > 30 AND city = &#39;Beijing&#39;;</pre></li><li><p> Check the user surnamed Zhang or Li:</p><pre class='brush:php;toolbar:false;'> SELECT * FROM users WHERE name LIKE &#39;Zhang%&#39; OR name LIKE &#39;Li%&#39;;</pre></li></ul><p> The rational use of <code>WHERE</code> can greatly improve query efficiency and reduce unnecessary data transmission.</p><h3 id="Sort-and-deduplication-ORDER-BY-and-DISTINCT"> Sort and deduplication: ORDER BY and DISTINCT</h3><p> The query data is in no order by default. If you want to sort it by a certain field, you can use <code>ORDER BY</code> :</p><pre class='brush:php;toolbar:false;'> SELECT name, age FROM users ORDER BY age DESC; -- Sort by descending age</pre><p> Sometimes there will be duplicate records in the data, especially after multiple tables are associated. You can use <code>DISTINCT</code> to deduplicate:</p><pre class='brush:php;toolbar:false;'> SELECT DISTINCT city FROM users;</pre><p> This statement returns all different city names.</p><p> These two features are often used together, such as you want to see users in all different cities and sort by city name:</p><pre class='brush:php;toolbar:false;'> SELECT DISTINCT city FROM users ORDER BY city ASC;</pre><h3 id="Aggregation-functions-and-grouping-statistics-GROUP-BY-and-aggregation-operations"> Aggregation functions and grouping statistics: GROUP BY and aggregation operations</h3><p> When you need statistics, such as the number of users in each city, the average age, etc., you need to use aggregate functions and <code>GROUP BY</code> .</p><p> Commonly used aggregate functions include:</p><ul><li> <code>COUNT()</code> : Count</li><li> <code>SUM()</code> : Sum</li><li> <code>AVG()</code> : Average</li><li> <code>MAX()</code> / <code>MIN()</code> : Maximum/minimum value</li></ul><p> For example, count the number of users in each city:</p><pre class='brush:php;toolbar:false;'> SELECT city, COUNT(*) AS user_count FROM users GROUP BY city;</pre><p> You can also combine <code>HAVING</code> to further filter the grouped results, such as only displaying cities with more than 5 users:</p><pre class='brush:php;toolbar:false;'> SELECT city, COUNT(*) AS user_count FROM users GROUP BY city HAVING COUNT(*) > 5;</pre><p> Although this part seems a bit complicated, it can complete many practical data analysis tasks after mastering it.</p> <p> Basically that's it. SQL's <code>SELECT</code> function is very powerful. The above mentioned is the most commonly used part. After mastering it, you can deal with most daily query scenarios.</p>

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