<p>In MySQL, the WHERE clause is used for data filtering and conditional querying. Its usage includes: 1. Basic usage: SELECT column1, column2 FROM table_name WHERE condition; 2. Logical operators: AND and OR are used to combine conditions; 3. Comparison operators and functions: such as BETWEEN, IN, LIKE, etc.; 4. Performance optimization: Avoid using functions in WHERE, pay attention to the consistency of condition order and data type. Mastering the WHERE clause can improve query efficiency and avoid data redundancy. </p> <p><img src="/static/imghw/default1.png" data-src="https://img.php.cn/upload/article/001/431/639/174773952284773.jpg" class="lazy" alt="The usage of where in mysql is detailed explanation of where condition query statement usage"></p> <p> In MySQL, the <code>WHERE</code> clause is an important tool for data filtering and querying. Let's take a closer look at the usage of <code>WHERE</code> and its application in conditional query.</p> <p> In database operations, the <code>WHERE</code> clause allows us to filter data based on specific conditions, which is particularly important for accurate queries under large data volumes. Mastering the usage of <code>WHERE</code> can not only improve query efficiency, but also avoid data redundancy and errors.</p> <p> Let's start with the most basic usage of <code>WHERE</code> :</p><pre class='brush:php;toolbar:false;'> SELECT column1, column2 FROM table_name WHERE condition;</pre><p> <code>condition</code> here can be any legal SQL expression for filtering records that meet the criteria. For example, if we want to filter out all employees whose wages are higher than 5,000 from the <code>employees</code> table, we can write this:</p><pre class='brush:php;toolbar:false;'> SELECT first_name, last_name, salary FROM employees WHERE salary > 5000;</pre><p> But <code>WHERE</code> 's charm is more than that. It supports multiple operators and functions, allowing us to build complex query conditions. Let's take a look at some more advanced usages:</p><p> First is the use of logical operators, such as <code>AND</code> and <code>OR</code> , which allow us to combine multiple conditions:</p><pre class='brush:php;toolbar:false;'> SELECT first_name, last_name, department FROM employees WHERE salary > 5000 AND department = &#39;IT&#39;;</pre><p> This query returns information about employees who have a salary of more than 5,000 in the IT department. It should be noted that <code>AND</code> has higher priority than <code>OR</code> . If you need to change the priority, you can use brackets:</p><pre class='brush:php;toolbar:false;'> SELECT first_name, last_name, department FROM employees WHERE (salary > 5000 AND department = &#39;IT&#39;) OR (salary > 7000 AND department = &#39;Sales&#39;);</pre><p> This query will return employees whose IT department has a salary of more than 5,000, or employees whose salary of more than 7,000 in the sales department.</p><p> <code>WHERE</code> also supports comparison operators such as <code>=</code> , <code>!=</code> , <code><</code> , <code>></code> , <code><=</code> , <code>>=</code> , as well as operators such as <code>BETWEEN</code> and <code>IN</code> :</p><pre class='brush:php;toolbar:false;'> SELECT product_name, price FROM products WHERE price BETWEEN 10 AND 50;</pre><p> This query returns all products with prices between 10 and 50.</p><pre class='brush:php;toolbar:false;'> SELECT customer_name FROM customers WHERE country IN (&#39;USA&#39;, &#39;Canada&#39;, &#39;Mexico&#39;);</pre><p> This query returns customers from the United States, Canada or Mexico.</p><p> Use <code>LIKE</code> and <code>NOT LIKE</code> to perform pattern matching queries:</p><pre class='brush:php;toolbar:false;'> SELECT first_name, last_name FROM employees WHERE last_name LIKE &#39;Joh%&#39;;</pre><p> This query returns all employees whose last name starts with 'Joh'.</p><p> In practical applications, the use of <code>WHERE</code> clauses requires attention to some performance optimization and best practices. For example, try to avoid using functions in <code>WHERE</code> clauses, as this may cause the inability to use indexes, thereby reducing query efficiency:</p><pre class='brush:php;toolbar:false;'> -- Avoid writing SELECT like this * FROM orders WHERE YEAR(order_date) = 2023; -- Change it to SELECT * FROM orders WHERE order_date >= &#39;2023-01-01&#39; AND order_date < &#39;2024-01-01&#39;;</pre><p> Another thing to note is that the order of <code>WHERE</code> conditions in complex queries may affect performance. In MySQL, the query optimizer tries to optimize queries, but sometimes manual adjustment of the order of conditions can lead to significant performance improvements.</p><p> When using the <code>WHERE</code> clause, you also need to pay attention to the consistency of the data type. For example, when comparing strings, if one side is a numerical type, it may lead to unexpected results:</p><pre class='brush:php;toolbar:false;'> -- There may be problems SELECT * FROM products WHERE product_code = 123; -- SELECT should be written like this * FROM products WHERE product_code = &#39;123&#39;;</pre><p> In general, the <code>WHERE</code> clause is a powerful tool in MySQL for data filtering and conditional query. By using various operators and functions reasonably, we can build complex and efficient queries. However, in practical applications, attention needs to be paid to performance optimization and best practices to ensure the efficiency and accuracy of the query.</p>

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