Commands to implement selection operations in sql

<blockquote><p>The command to implement selection operation in SQL is SELECT, which specifies conditions through the WHERE clause to extract rows that meet the conditions. Conditions consist of comparison operators, logical operators, and aggregate functions that are used to compare values, combine conditions, and calculate values. </p></blockquote> <p><img src="https://img.php.cn/upload/article/202405/02/2024050200541816722.jpg" alt="Commands to implement selection operations in sql" ></p> <p><strong>Commands to implement selection operations in SQL</strong></p> <p>In SQL, selection operations are used to extract matching values ​​from a table Rows with specific conditions. The command that implements the selection operation is SELECT. </p> <p><strong>SELECT statement syntax</strong></p> <pre class="brush:php;toolbar:false"><code class="sql">SELECT [列名1, 列名2, ...] FROM [表名] WHERE [条件]</code></pre> <p><strong>WHERE clause</strong></p> <p>The WHERE clause specifies the conditions by which rows are selected. Conditions in the WHERE clause can consist of comparison operators, logical operators, and aggregate functions. </p> <p><strong>Comparison operators</strong></p> <p>Comparison operators are used to compare two values ​​and return a Boolean value (true or false). Commonly used comparison operators include: </p> <ul> <li> <code>=</code>: equal to </li> <li> <code><></code> or <code>!=</code> : Not equal to </li> <li> <code><</code>: Less than </li><li><code>></code>: Greater than </li> <li> <code><=</code> : Less than or equal to </li><li><code>>=</code> : Greater than or equal to </li> </ul> <p>##Logical operator<strong></strong></p>Logical operator Used to combine multiple conditions together. Commonly used logical operators include: <p></p> <ul> <li>AND<code>: If both conditions are true, return true; otherwise, return false. </code> </li> <li>OR<code>: Returns true if at least one condition is true; otherwise, returns false. </code> </li> <li>NOT<code>: Inverts the Boolean value of a condition (true becomes false, false becomes true). </code> </li> </ul> <p>Aggregation functions<strong></strong></p>Aggregation functions calculate a range of values ​​in a group and return a single value. Commonly used aggregate functions include: <p></p> <ul> <li>#COUNT()<code>: Counts the number of rows in a group. </code> </li> <li>SUM()<code>: Calculate the sum of the values ​​in the group. </code> </li> <li>AVG()<code>: Calculate the average of the values ​​in the group. </code> </li> <li>MAX()<code>: Calculate the maximum value in the group. </code> </li> <li>MIN()<code>: Calculate the minimum value in the group. </code> </li> </ul> <p>Example<strong></strong></p>Select all customers older than 30 from a table named "customers": <p></p> <pre class="brush:php;toolbar:false"><code class="sql">SELECT * FROM customers WHERE age > 30;</code></pre>

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