java - java中的嵌套选择

Question

我想将下面代码中的 2 个选择合并到具有嵌套选择的单个查询中。我怎样才能实现这一目标？

这是我的代码

  int idst1 = // some Integer value

  String SQLST1 = "select idl from " +
                  "studentlesson " +
                  "WHERE id=" + idst1;

  rs = stmt.executeQuery(SQLST1);
  rs.first();
  id1 = rs.getInt(1);

  String curses = "SELECT namel FROM lesson where idl=" + id1;
  rs = stmt1.executeQuery(curses);
  rs.first();

  System.out.println("nxt:" + rs.next());
  String s = rs.getString(7);
  System.out.println(idst1 + ", " + s + ", ");

Answer 1

假设我正确理解并且您的表格如下:

LESSON
------
name1
id1

STUDENTLESSON
-------------
id
id1

其中 LESSON.id1 与 STUDENTLESSON.id1

您可以使用以下查询:

SELECT name1 FROM LESSON, STUDENTLESSON
WHERE LESSON.id1 = STUDENTLESSON.id1
AND STUDENTLESSON.id = idst1

或者写成正确的 JOIN:

SELECT LESSON.name1
FROM LESSON
INNER JOIN STUDENTLESSON ON LESSON.id1=STUDENTLESSON.id1
WHERE id1 = idst1;