单表查询

在 Java 语言当中，可以通过声明一个 Author 类来定义如何存放作者的基础信息，根据数据的类型和取值范围从 Java 语言当中选择合适的数据类型来存放对应的数据，例如：

• 使用 Int 类型变量存放 int 类型的数据。
• 使用 Long 类型变量存放 bigint 类型的数据。
• 使用 Short 类型变量存放 tinyint 类型的数据。
• 使用 String 类型变量存放 varchar 类型的数据。
• ...

public class Author {
    private Long id;
    private String name;
    private Short gender;
    private Short birthYear;
    private Short deathYear;

    public Author() {}

     // Skip the getters and setters.
}

public class AuthorDAO {

    // Omit initialization of instance variables.

    public List<Author> getAuthors() throws SQLException {
        List<Author> authors = new ArrayList<>();

        try (Connection conn = ds.getConnection()) {
            Statement stmt = conn.createStatement();
            ResultSet rs = stmt.executeQuery("SELECT id, name FROM authors");
            while (rs.next()) {
                Author author = new Author();
                author.setId(rs.getLong("id"));
                author.setName(rs.getString("name"));
                authors.add(author);
            }
        }
        return authors;
    }
}

• 在获得数据库连接之后，你可以通过 conn.createStatement() 语句创建一个 Statement 实例对象。
• 然后调用 stmt.executeQuery("query_sql") 方法向 TiDB 发起一个数据库查询请求。
• 数据库返回的查询结果将会存放到 ResultSet 当中，通过遍历 ResultSet 对象可以将返回结果映射到此前准备的 Author 类对象当中。

对于 Java 程序而言，可以通过同一个 SQL 来处理带有动态参数的数据查询请求。

将参数拼接到 SQL 语句当中也许是一种方法，但是这可能不是一个好的主意，因为这会给应用程序带来潜在的 SQL 注入风险。

在处理这类查询时，应该使用 PreparedStatement 来替代普通的 Statement。

public List<Author> getAuthorsByBirthYear(Short birthYear) throws SQLException {
    List<Author> authors = new ArrayList<>();
    try (Connection conn = ds.getConnection()) {
        PreparedStatement stmt = conn.prepareStatement("""
        SELECT * FROM authors WHERE birth_year = ?;
        """);
        stmt.setShort(1, birthYear);
        ResultSet rs = stmt.executeQuery();
        while (rs.next()) {
            Author author = new Author();
            author.setId(rs.getLong("id"));
            author.setName(rs.getString("name"));
            authors.add(author);
        }
    }
    return authors;
}

public List<Author> getAuthorsSortByBirthYear() throws SQLException {
    List<Author> authors = new ArrayList<>();
    try (Connection conn = ds.getConnection()) {
        Statement stmt = conn.createStatement();
        ResultSet rs = stmt.executeQuery("""
            SELECT id, name, birth_year
            FROM authors
            ORDER BY birth_year DESC;
            """);

        while (rs.next()) {
            Author author = new Author();
            author.setId(rs.getLong("id"));
            author.setName(rs.getString("name"));
            author.setBirthYear(rs.getShort("birth_year"));
            authors.add(author);
        }
    }
    return authors;
}

public List<Author> getAuthorsWithLimit(Integer limit) throws SQLException {
    List<Author> authors = new ArrayList<>();
    try (Connection conn = ds.getConnection()) {
        PreparedStatement stmt = conn.prepareStatement("""
            SELECT id, name, birth_year
            FROM authors
            ORDER BY birth_year DESC
            LIMIT ?;
            """);
        stmt.setInt(1, limit);
        ResultSet rs = stmt.executeQuery();
        while (rs.next()) {
            Author author = new Author();
            author.setId(rs.getLong("id"));
            author.setName(rs.getString("name"));
            author.setBirthYear(rs.getShort("birth_year"));
            authors.add(author);
        }
    }
    return authors;
}

public class AuthorCount {
    private Short birthYear;
    private Integer authorCount;

    public AuthorCount() {}

     // Skip the getters and setters.
}

public List<AuthorCount> getAuthorCountsByBirthYear() throws SQLException {
    List<AuthorCount> authorCounts = new ArrayList<>();
    try (Connection conn = ds.getConnection()) {
        Statement stmt = conn.createStatement();
        ResultSet rs = stmt.executeQuery("""
            SELECT birth_year, COUNT(DISTINCT id) AS author_count
            FROM authors
            GROUP BY birth_year
            ORDER BY author_count DESC;
            """);

        while (rs.next()) {
            AuthorCount authorCount = new AuthorCount();
            authorCount.setBirthYear(rs.getShort("birth_year"));
            authorCount.setAuthorCount(rs.getInt("author_count"));
            authorCounts.add(authorCount);
        }
    }
    return authorCount;
}