搜索功能的 SQL 查询

Question

我想做一个包含多个 Java Swing 组件的搜索功能，用户可以通过（姓名/国籍/专业/经验）进行搜索，结果将显示在 Jtable 中。

我只在 SQL 查询中苦苦挣扎，好像用户只输入了一个“名称”，不会检索任何数据，因为它像这样（名称、空、空、空）进入数据库而我没有我的数据库中有任何空值。

所以我想检索具有该名称的所有数据，而不考虑其他列，但同时，如果他们还选择了特定专业，例如，我想检索具有所选名称和专业的所有数据，依此类推.

希望你能理解我的问题。

我当前的 SQL 语句：

public ArrayList<Applications> getData(String name, String nationality, String specialty, String experience) {

   ArrayList<Applications> list = new ArrayList<Applications>();
   Connection con = getConnection();
   Statement st;
   ResultSet rows;

   try {
       st = con.createStatement();

       rows = st.executeQuery("SELECT * FROM applications WHERE name LIKE '%" +  name+"%'"
            + " AND (nationality LIKE '" +  nationality+"')"
            + " AND (specialty LIKE '" +  specialty+"')"
            + " AND (experience LIKE '" +  experience+"')");

       Applications applications;

       while(rows.next()) {
           applications = new Applications(
               rows.getInt("id"),
               rows.getString("name"),
               rows.getString("nationality"),
               rows.getString("Specialty"),
               rows.getString("experience")
           );

          list.add(applications);
       }   
   } catch (SQLException ex) {
       Logger.getLogger(MyQuery.class.getName()).log(Level.SEVERE, null, ex);
   }

   return list;
}

Answer 1

让我们看看您的查询：

   rows = st.executeQuery("SELECT * FROM applications WHERE name LIKE '%" +  name+"%'"
        + " AND (nationality LIKE '" +  nationality+"')"
        + " AND (specialty LIKE '" +  specialty+"')"
        + " AND (experience LIKE '" +  experience+"')");

这里，由于只给出了name，所以其他值为null。如果您出于测试目的编写此代码：

String foo = null;
System.out.println(foo + "");

输出将是

“空”

所以，由于您的值是null，生成的查询将是

   SELECT * FROM applications WHERE name LIKE '%Rowan Atkinson%'
        AND (nationality LIKE 'null')
        AND (specialty LIKE 'null')
        AND (experience LIKE 'null')

首先，让我们确保您在null 的情况下为空String：

   rows = st.executeQuery("SELECT * FROM applications WHERE name LIKE '%" +  ((name == null) ? "" : name)+"%'"
        + " AND (nationality LIKE '" +  ((nationality == null) ? "" : nationality)+"')"
        + " AND (specialty LIKE '" +  ((specialty == null) ? "" : specialty)+"')"
        + " AND (experience LIKE '" +  ((experience == null) ? "" : experience)+"')");

下一个问题是您只将% 放在name 上，这也是不正确的，所以让我们修复它：

   rows = st.executeQuery("SELECT * FROM applications WHERE name LIKE '%" +  ((name == null) ? "" : name)+"%'"
        + " AND (nationality LIKE '%" +  ((nationality == null) ? "" : nationality)+"%')"
        + " AND (specialty LIKE '%" +  ((specialty == null) ? "" : specialty)+"%')"
        + " AND (experience LIKE '%" +  ((experience == null) ? "" : experience)+"%')");

现在阅读 YCF_L 的答案，以便您将命名参数用于 PreparedStatement。

Answer 2

下面的代码可能会给你预期的结果。

StringBuilder sql=new StringBuilder("SELECT * FROM applications WHERE 1=1 ");
        if(!name.isEmpty()){
            sql.append(" AND name LIKE '%" +  name+"%'");
        }
        if(!nationality.isEmpty()){
            sql.append(" AND (nationality LIKE '" +  nationality+"')");
        }
        if(!specialty.isEmpty()){
            sql.append(" AND (specialty LIKE '" +  specialty+"')");
        }
        if(!experience.isEmpty()){
            sql.append(" AND (experience LIKE '" +  experience+"')");
        }

        rows = st.executeQuery(sql.toString());

试试这个

Answer 3

在这种情况下，您有两个解决方案：

1. 检查值，如果不为空，则将该部分附加到查询中，否则忽略它
2. 第二种解决方案，您可以检查查询中的值是否为空。

我会给你一个关于第二个的例子，你可以使用NamedParameterStatement 它就像PrepapredStatement 以避免语法错误和SQL注入：

String query = "SELECT * FROM applications"
        + " WHERE (:name is null or name LIKE CONCAT('%', :name, '%')) "
        + " AND (:nationality is null or nationality LIKE :nationality)"
        + " AND (:specialty is null or specialty LIKE :specialty)"
        + " AND (:experience is null or experience LIKE :experience)";

NamedParameterStatement namedParametterStatement = 
                           new NamedParameterStatement(connection, query);

namedParametterStatement.setString("name", name);
namedParametterStatement.setString("nationality", nationality);
namedParametterStatement.setString("specialty", specialty);
namedParametterStatement.setString("experience", experience);

请注意这里的技巧：

(:nationality is null or nationality LIKE :nationality)

它将检查nationality 是否为空或您传递的值之类的国籍，这样您就避免了空。

---

一些很好的参考：

• Named Parameters for PreparedStatement