How to query the database in java
How to query the database in java: first create the user and teacher databases; then establish a one-to-many connection between the [user_id] column of the teacher table and the id column of the user table; then add data to the user data table, and Query the user database data according to the conditions; finally query the slave table data according to the main table.
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How to query the database in java:
1. Create database
Create user database
Create teacher database
The user_id
column of the teacher table establishes a one-to-many connection with the id column of the user table, using user_id as a foreign key.
2. Java programming to query the database
Add data to the user data table
/** * 添加数据 */ @Test public void addData() { Connection connection = null; PreparedStatement pstmt =null; try { connection = JDBCUtils_V3.getConnection(); String sql = "insert into user values(null,?,?)"; pstmt = connection.prepareStatement(sql); pstmt.setString(1, "wangxuan"); pstmt.setString(2, "741852"); int row = pstmt.executeUpdate(); if (row>0) { System.out.println("数据添加成功!"); }else { System.out.println("数据添加失败!"); } } catch (SQLException e) { // TODO Auto-generated catch block e.printStackTrace(); }finally{ JDBCUtils_V3.release(connection, pstmt, null); } }
Query according to conditions user database data
/** * 按照条件查询数据 */ @Test public void selectTest() { Connection conn = null; PreparedStatement pstmt = null; ResultSet rs =null; try { conn = JDBCUtils_V3.getConnection(); String sql = "select * from user where password = ?"; pstmt = conn.prepareStatement(sql); pstmt.setString(1, "123456"); rs = pstmt.executeQuery(); while (rs.next()) { System.out.println(rs.getString(1)+"----"+rs.getString(2)+"---"+rs.getString(3)); } // System.out.println(rs); } catch (SQLException e) { // TODO Auto-generated catch block e.printStackTrace(); }finally{ JDBCUtils_V3.release(conn, pstmt, rs); } }
One-to-many query/Query the slave table teacher data based on the main table user
/** * 一对多查询 * 根据主表查询从表 */ @Test public void selectOnetoMore() { Connection conn = null; PreparedStatement pstmt = null; ResultSet rs = null; try { conn = JDBCUtils_V3.getConnection(); // String sql = "select * from teacher where user_id = (select id from user where username =?) "; String sql = "select * from user,teacher where user.id = teacher.user_id "; pstmt = conn.prepareStatement(sql); // pstmt.setString(1, "wangxuan"); rs = pstmt.executeQuery(); while (rs.next()) { // System.out.println(rs.getString(1)+"----"+rs.getString(2)+"---"+rs.getString(3)+"---"+rs.getString(4)); System.out.println(rs.getString(1)+"----"+rs.getString(2)+"---"+rs.getString(3)+"---"+rs.getString(4)+"----"+rs.getString(5)+"----"+rs.getString(6)+"----"+rs.getString(7)); } System.out.println("查询完成"); } catch (SQLException e) { // TODO Auto-generated catch block e.printStackTrace(); }finally{ JDBCUtils_V3.release(conn, pstmt, rs); } }
One-to-many query/Query the main table based on the slave table
/** * 一对多查询 * 根据从表查询主表数据 */ @Test public void selectMoretoOne() { Connection connection = null; PreparedStatement pstmtPreparedStatement = null; ResultSet rSet =null; try { connection = JDBCUtils_V3.getConnection(); String sql = "select * from user where id = (select user_id from teacher where teacher=?)"; pstmtPreparedStatement = connection.prepareStatement(sql); pstmtPreparedStatement.setString(1, "钱田"); rSet = pstmtPreparedStatement.executeQuery(); while (rSet.next()) { System.out.println(rSet.getString(1)+"----"+rSet.getString(2)+"---"+rSet.getString(3)); } } catch (SQLException e) { // TODO Auto-generated catch block e.printStackTrace(); }finally{ JDBCUtils_V3.release(connection, pstmtPreparedStatement, rSet); } } }
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