如何從MySQL中的表中選擇數據？

<p>要從MySQL表中選擇數據，應使用SELECT語句，1. 使用SELECT column1, column2 FROM table_name獲取指定列，或使用SELECT *獲取所有列；2. 使用WHERE子句過濾行，如SELECT name, age FROM users WHERE age > 25；3. 使用ORDER BY對結果排序，如ORDER BY age DESC表示按年齡降序排列；4. 使用LIMIT限制返回行數，如LIMIT 5返回前5行，或使用LIMIT 10 OFFSET 20實現分頁；5. 使用AND、OR和括號組合多個條件，如WHERE age > 18 AND city = 'New York'；6. 使用LIKE進行模式匹配，如LIKE 'A%'查找以A開頭的名稱；7. 為提升性能，應僅選擇必要列、在WHERE和ORDER BY列上建立索引，並在大表上先用LIMIT測試查詢，最終通過逐步添加WHERE、ORDER BY和LIMIT構建完整查詢。 </p> <p><img src="/static/imghw/default1.png" data-src="https://img.php.cn/upload/article/000/000/000/175558242240385.jpg" class="lazy" alt="How to select data from a table in MySQL?"></p> <p> To select data from a table in MySQL, you use the <code>SELECT</code> statement. This is one of the most common and essential SQL commands for retrieving data from a database.</p> <h3 id="Basic-Syntax"> Basic Syntax</h3><pre class='brush:php;toolbar:false;'> SELECT column1, column2, ... FROM table_name;</pre><ul><li> Replace <code>column1, column2, ...</code> with the actual column names you want to retrieve.</li><li> Use <code>*</code> if you want to select all columns.</li></ul><h4 id="Example-Select-Specific-Columns"> Example: Select Specific Columns</h4><pre class='brush:php;toolbar:false;'> SELECT name, email FROM users;</pre><p> This retrieves only the <code>name</code> and <code>email</code> columns from the <code>users</code> table.</p><h4 id="Example-Select-All-Columns"> Example: Select All Columns</h4><pre class='brush:php;toolbar:false;'> SELECT * FROM users;</pre><p> This returns every column in the <code>users</code> table.</p><h3 id="Filter-Results-with-WHERE"> Filter Results with WHERE</h3><p> To get specific rows that meet certain conditions, use the <code>WHERE</code> clause.</p><pre class='brush:php;toolbar:false;'> SELECT name, age FROM users WHERE age > 25;</pre><p> This selects the name and age of users older than 25.</p><p> Common operators in <code>WHERE</code> :</p><ul><li> <code>=</code> (equal)</li><li> <code><></code> or <code>!=</code> (not equal)</li><li> <code><</code> , <code>></code> , <code><=</code> , <code>>=</code> (comparison)</li><li> <code>LIKE</code> (pattern matching)</li><li> <code>IN</code> (match any value in a list)</li><li> <code>BETWEEN</code> (range check)</li></ul><h4 id="Example-with-LIKE"> Example with LIKE</h4><pre class='brush:php;toolbar:false;'> SELECT * FROM users WHERE name LIKE &#39;A%&#39;;</pre><p> Finds users whose names start with "A".</p><h3 id="Sort-Results-with-ORDER-BY"> Sort Results with ORDER BY</h3><p> To sort the output, use <code>ORDER BY</code> .</p><pre class='brush:php;toolbar:false;'> SELECT name, age FROM users ORDER BY age DESC;</pre><p> Sorts results by age in descending order. Use <code>ASC</code> for ascending (default).</p><h3 id="Limit-Number-of-Rows"> Limit Number of Rows</h3><p> Use <code>LIMIT</code> to restrict how many rows are returned.</p><pre class='brush:php;toolbar:false;'> SELECT * FROM users LIMIT 5;</pre><p> Returns only the first 5 rows.</p><p> You can also paginate:</p><pre class='brush:php;toolbar:false;'> SELECT * FROM users LIMIT 10 OFFSET 20;</pre><p> Skips 20 rows and returns the next 10.</p><h3 id="Combine-Conditions"> Combine Conditions</h3><p> Use <code>AND</code> , <code>OR</code> , and parentheses to build complex filters.</p><pre class='brush:php;toolbar:false;'> SELECT * FROM users WHERE age > 18 AND city = &#39;New York&#39;;</pre><h3 id="Key-Tips"> Key Tips</h3> <ul> <li> Always specify only the columns you need instead of using <code>SELECT *</code> when possible — it's more efficient.</li> <li> Use <code>WHERE</code> to avoid pulling unnecessary data.</li> <li> Add indexes on columns used in <code>WHERE</code> , <code>ORDER BY</code> , or <code>JOIN</code> clauses for better performance.</li> <li> Test your queries with <code>LIMIT</code> first when working with large tables.</li> </ul> <p> Basically, start simple with <code>SELECT ... FROM ...</code> , then add <code>WHERE</code> , <code>ORDER BY</code> , and <code>LIMIT</code> as needed.</p>

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