php，$sql = "SELECT field1, field2, field3 FROM table_name";，``php，$sql = "SELECT field1, field2, field3 FROM table_name";，``php，$pdo = new PDO('mysql:host=localhost;dbname=test', 'username', 'password');，$stmt = $pdo>prepare('SELECT * FROM users WHERE age > ? AND status = ?');，$stmt>execute([25, 'active']);，$results = $stmt>fetchAll();，`，，这段代码会从users`表中查询年龄大于25且状态为'active'的所有记录。php，，``php，$sql = "SELECT * FROM users WHERE age > 18";，$result = $conn>query($sql);，`，，这段代码会从users`表中筛选出年龄大于18岁的用户。，SELECT * FROM 表1 WHERE 字段1 IN (SELECT 字段2 FROM 表2)，``SqlDataAdapter 和 DataTable 来执行查询并返回结果。以下是一个示例代码：，，``csharp，using System.Data;，using System.Data.SqlClient;，，string connectionString = "your_connection_string";，string query = "SELECT * FROM your_table";，，using (SqlConnection connection = new SqlConnection(connectionString))，{， SqlDataAdapter adapter = new SqlDataAdapter(query, connection);， DataTable dataTable = new DataTable();， adapter.Fill(dataTable);，}，``sql，SELECT column1, column2, COUNT(*)，FROM table_name，GROUP BY column1, column2，HAVING COUNT(*) > 1;，``，，这段代码会返回在column1和column2上具有相同值的记录，并且这些记录的数量大于1。GROUP BY和HAVING子句。要查找某个表中重复的记录，可以使用以下SQL语句：，，``sql，SELECT column1, column2, COUNT(*)，FROM table_name，GROUP BY column1, column2，HAVING COUNT(*) > 1;，`，，这里，column1和column2是你想要检查是否重复的列，table_name是表的名称。这个查询将返回所有在column1和column2`组合上出现超过一次的记录。Powered By Z-BlogPHP 1.7.3
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