比较两个表，得到不同的打印输出。

Question

我的MySQL中有两个表，它们在列方面完全相同，我们可以将它们称为id、name、path、date。现在，我想对这两个表进行比较，以便得到表1与表2的区别，以及表2与表1的不同之处。我只想在我的Java程序中打印它，现在，一个简单的差异的System.out.print将是完美的。

我只想检查两个表中name列中条目的差异。任何帮助都是非常感谢的！

Answer 1

如果您只想看到table1中存在但在table2中不存在的名称列表，请尝试以下操作：

SELECT a.name
FROM table1 a
LEFT JOIN table2 b ON a.name = b.name
WHERE table2.name IS NULL

交换查询中的table1和table2，以获取table2中不匹配的名称。

Answer 2

这将连接id上的两个表，并显示名称不同的行：

select a.name, b.name 
from table1 a 
join table2 b on a.id = b.id
where a.name <> b.name

Answer 3

reaanb答复是正确的，应该被接受。

下面是Java代码示例，它显示了他的SQL代码。我不使用MySQL (如问题中所示)。相反，本例使用H2数据库引擎，创建一个临时内存数据库。

try {
    Class.forName ( "org.h2.Driver" );
} catch ( final ClassNotFoundException e ) {
    e.printStackTrace ( );
}
try ( Connection conn = DriverManager.getConnection ( "jdbc:h2:mem:trash_me_db" ) ;
      Statement stmt = conn.createStatement ( ) ) {
    String sql = null;

    // Table 1
    sql = "CREATE TABLE t1_ (\n" +
            "  id_ INT IDENTITY ,\n" +
            "  name_ VARCHAR \n" +
            ");";
    stmt.executeUpdate ( sql );
    stmt.execute ( "INSERT INTO t1_ ( name_ ) VALUES ('Jesse');" );
    stmt.execute ( "INSERT INTO t1_ ( name_ ) VALUES ('Wendy');" );
    stmt.execute ( "INSERT INTO t1_ ( name_ ) VALUES ('Lisa');" );
    stmt.execute ( "INSERT INTO t1_ ( name_ ) VALUES ('Susan');" );
    stmt.execute ( "INSERT INTO t1_ ( name_ ) VALUES ('Jimmy');" );
    stmt.execute ( "INSERT INTO t1_ ( name_ ) VALUES ('Wine');" );

    // Dump
    System.out.println ( "\nDump table t1_" );
    try ( ResultSet rs = stmt.executeQuery ( "SELECT * FROM t1_ ;" ) ) {
        while ( rs.next ( ) ) {
            final int id = rs.getInt ( "id_" );    //Retrieve by column name
            final String name = rs.getString ( "name_" );
            System.out.println ( "id: " + id + " | name: " + name );
        }
    }

    // Table 2
    sql = "CREATE TABLE t2_ (\n" +
            "  id_ INT IDENTITY ,\n" +
            "  name_ VARCHAR \n" +
            ");";
    stmt.executeUpdate ( sql );
    stmt.execute ( "INSERT INTO t2_ ( name_ ) VALUES ('Jesse');" );
    stmt.execute ( "INSERT INTO t2_ ( name_ ) VALUES ('Wendy');" );
    stmt.execute ( "INSERT INTO t2_ ( name_ ) VALUES ('Lisa');" );
    stmt.execute ( "INSERT INTO t2_ ( name_ ) VALUES ('Susan');" );
    stmt.execute ( "INSERT INTO t2_ ( name_ ) VALUES ('Jimmy');" );
    stmt.execute ( "INSERT INTO t2_ ( name_ ) VALUES ('Beer');" );

    // Dump
    System.out.println ( "\nDump table t2_" );
    try ( ResultSet rs = stmt.executeQuery ( "SELECT * FROM t2_ ;" ) ) {
        while ( rs.next ( ) ) {
            final int id = rs.getInt ( "id_" );    //Retrieve by column name
            final String name = rs.getString ( "name_" );
            System.out.println ( "id: " + id + " | name: " + name );
        }
    }

    // Difference, left.
    System.out.println ( "\nShow names that exist in t1_ but not in t2_." );
    sql = "SELECT *\n" +
            "FROM t1_ \n" +
            "LEFT JOIN t2_ ON t1_.name_ = t2_.name_\n" +
            "WHERE t2_.name_ IS NULL";
    try ( ResultSet rs = stmt.executeQuery ( sql ) ) {
        while ( rs.next ( ) ) {
            final int id = rs.getInt ( "id_" );    //Retrieve by column name
            final String name = rs.getString ( "name_" );
            System.out.println ( "id: " + id + " | name: " + name );
        }
    }

    // Difference, right.
    System.out.println ( "\nShow names that exist in t2_ but not in t1_." );
    sql = "SELECT *\n" +
            "FROM t2_ \n" +
            "LEFT JOIN t1_ ON t2_.name_ = t1_.name_\n" +
            "WHERE t1_.name_ IS NULL";
    try ( ResultSet rs = stmt.executeQuery ( sql ) ) {
        while ( rs.next ( ) ) {
            final int id = rs.getInt ( "id_" );    //Retrieve by column name
            final String name = rs.getString ( "name_" );
            System.out.println ( "id: " + id + " | name: " + name );
        }
    }


} catch ( final SQLException e ) {
    e.printStackTrace ( );
}

跑的时候。

Dump table t1_
id: 1 | name: Jesse
id: 2 | name: Wendy
id: 3 | name: Lisa
id: 4 | name: Susan
id: 5 | name: Jimmy
id: 6 | name: Wine

Dump table t2_
id: 1 | name: Jesse
id: 2 | name: Wendy
id: 3 | name: Lisa
id: 4 | name: Susan
id: 5 | name: Jimmy
id: 6 | name: Beer

Show names that exist in t1_ but not in t2_.
id: 6 | name: Wine

Show names that exist in t2_ but not in t1_.
id: 6 | name: Beer