How to use conditions to filter data in SQL

<p><img src="/static/imghw/default1.png" data-src="https://img.php.cn/upload/article/000/887/227/170823261547012.jpg" class="lazy" alt="How to use conditions to filter data in SQL"></p> <p>The usage of WHERE in SQL requires specific code examples</p> <p>SQL (Structured Query Language) is a standardized language used to manage relational database management systems (RDBMS) . In SQL, the WHERE clause is used to filter the data returned by the SELECT statement. Through the WHERE clause, we can select the required data rows based on specific conditions. </p> <p>The general syntax of the WHERE clause is as follows: </p><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class='brush:php;toolbar:false;'>SELECT column1, column2, ... FROM table_name WHERE condition;</pre><div class="contentsignin">Copy after login</div></div><p>In the above syntax, <code>column1, column2, ...</code> are the columns you want to select, <code>table_name</code> is the table you want to select data from, <code>condition</code> is the condition for filtering data. </p><p>Some common examples will be given below to illustrate the usage of WHERE. </p><ol><li>Simple equal operator (<code>=</code>): </li></ol><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class='brush:php;toolbar:false;'>SELECT * FROM customers WHERE city = 'New York';</pre><div class="contentsignin">Copy after login</div></div><p>The above code will select all locations from the <code>customers</code> table New York customers. </p><ol start="2"><li>Not equal to operator (<code><></code>): </li></ol><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class='brush:php;toolbar:false;'>SELECT * FROM employees WHERE department <> 'HR';</pre><div class="contentsignin">Copy after login</div></div><p>The above code will be from the <code>employees</code> table Select all employees whose department is not HR. </p><ol start="3"><li>Greater than operator (<code>></code>): </li></ol><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class='brush:php;toolbar:false;'>SELECT * FROM products WHERE price > 100;</pre><div class="contentsignin">Copy after login</div></div><p>The above code will select the price greater than 100 from the <code>products</code> table of all products. </p><ol start="4"><li>Less than operator (<code><</code>): </li></ol><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class='brush:php;toolbar:false;'>SELECT * FROM orders WHERE order_date < '2021-01-01';</pre><div class="contentsignin">Copy after login</div></div><p>The above code will select the order date earlier from the <code>orders</code> table All orders placed on January 1, 2021. </p><ol start="5"><li> Greater than or equal to operator (<code>>=</code>): </li></ol><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class='brush:php;toolbar:false;'>SELECT * FROM customers WHERE age >= 18;</pre><div class="contentsignin">Copy after login</div></div><p>The above code will select the age from the <code>customers</code> table All customers aged 18 and above. </p><ol start="6"><li> Less than or equal to operator (<code><=</code>): </li></ol><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class='brush:php;toolbar:false;'>SELECT * FROM employees WHERE hire_date <= '2019-01-01';</pre><div class="contentsignin">Copy after login</div></div><p>The above code will select onboarding from the <code>employees</code> table All employees with a date before or equal to January 1, 2019. </p><ol start="7"><li>BETWEEN Operator: </li></ol><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class='brush:php;toolbar:false;'>SELECT * FROM products WHERE price BETWEEN 50 AND 100;</pre><div class="contentsignin">Copy after login</div></div><p>The above code will select all products with a price between 50 and 100 from the <code>products</code> table. </p><ol start="8"><li>LIKE and wildcard operators: </li></ol><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class='brush:php;toolbar:false;'>SELECT * FROM customers WHERE last_name LIKE 'Sm%';</pre><div class="contentsignin">Copy after login</div></div><p>The above code will select all customers whose last name starts with "Sm" from the <code>customers</code> table. The wildcard character <code>%</code> represents any character. </p><ol start="9"><li>IN Operator: </li></ol><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class='brush:php;toolbar:false;'>SELECT * FROM employees WHERE department IN ('HR', 'Sales');</pre><div class="contentsignin">Copy after login</div></div><p>The above code will select all employees whose department is HR or Sales from the <code>employees</code> table. </p> <p>The above are sample codes for some common WHERE clauses. Through these examples, you can better understand the usage of WHERE clause in SQL queries. According to actual needs, you can write WHERE clauses based on different conditions to obtain the required data rows. </p>

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