我正在尝试在我的 java web 应用程序中运行以下查询：
SELECT platform AS "Platform" FROM edb.cases
该网络应用程序工作正常并且能够执行所有查询，但是每当我使用别名（通过“AS”）时，生成的数据集都会为我提供别名的空值。事实上，尽管在上面的查询中使用了“platform”列的别名，但生成的数据集的“Platform”键为空值，但为我提供了“platform”键的正确值（这是原始数据集）列的名称）。
现在，我需要执行的实际 sql 语句有点复杂，其中包含 select 语句，并使用别名在同一个表上进行两次左连接，如下所示：
SELECT numOne.platform , numTwo.platform AS 'PlatformTwo' FROM edb.cases LEFT JOIN
edb.platform as numOne ON (numOne.rank = cases.platform) LEFT JOIN edb.platform as numTwo ON
(numTwo.rank = cases.highestPlatform) WHERE cases.index = 1000
问题在于，生成的数据集包含键“platform”（对于 numOne 表）的正确值，但键“PlatformOne”和“PlatformTwo”不存在。别名不起作用！
我已经在 MySql 工作台中尝试了这两个语句，它们工作得很好。
请随时询问更多信息。
EDIT:
准备查询并将其发送到数据库的代码：
public static List<Map<String, Object>> executeQuery(final String query,
Map<Integer, Object> data) {
List<Map<String, Object>> result = null;
try {
Connection conn = createConnection();
PreparedStatement pstmt = null;
pstmt = conn.prepareStatement(query);
if(data != null) {
pstmt = createPreparedStatement(pstmt, data);
}
System.out.println(pstmt.toString());
//The GET_CASE_FOR_INDEX query uses the executequery function in the else block:
if((pstmt.toString().indexOf("INSERT") >= 0) || (pstmt.toString().indexOf("UPDATE") >= 0)) {
pstmt.executeUpdate();
} else {
ResultSet rs = pstmt.executeQuery();
ResultSetMetaData md = rs.getMetaData();
int columns = md.getColumnCount();
result = new ArrayList<Map<String, Object>>();
/*
* Get the next row of the ResultSet 'rs' and insert a Map of the Column/Value pair
* into the ArrayList of Maps 'result'
*/
while(rs.next()) {
Map<String, Object> row = new HashMap<String, Object>(columns);
for(int i=1; i <= columns; i++) {
try {
row.put(md.getColumnName(i), rs.getObject(i));
} catch(Exception e) {
System.out.println(md.getColumnName(i));
System.out.println(row);
e.printStackTrace();
}
}
result.add(row);
}
}
destroyConnection(conn);
pstmt.close();
} catch(SQLException e) {
//TODO
e.printStackTrace();
}
return result;
}
创建准备语句的函数：
//creates a prepared statement by checking the type of the value that needs to be set.
private static PreparedStatement createPreparedStatement(
PreparedStatement pstmt, Map<Integer, Object> data) {
try {
for(Integer key : data.keySet()) {
Object value = data.get(key);
System.out.println(key);
if(data.get(key).equals(Types.NULL)) {
pstmt.setNull(key, Types.INTEGER);
} else if(value.getClass().equals(Integer.class)) {
pstmt.setInt(key, (Integer) value);
} else if(value.getClass().equals(String.class)) {
pstmt.setString(key, (String) value);
} else if(value.getClass().equals(Date.class)) {
pstmt.setDate(key, (Date) value);
} else if(value.getClass().equals(Timestamp.class)) {
pstmt.setTimestamp(key, (Timestamp) value);
}
}
}catch (SQLException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
return pstmt;
}
使用executeQuery函数执行查询并将其发送到Web模板的代码片段：
Map<Integer, Object> data_details = new HashMap<Integer, Object>();
data_details.put(1, parameter_ID);
List<Map<String, Object>> details = DBUtility.executeQuery(DBQuery.GET_CASE_FOR_INDEX, data_details);
webContext.setVariable("details", details);//This is where the template variable is being set
System.out.println(details);
GET_CASE_FOR_INDEX 查询是：
SELECT numOne.platform , numTwo.platform AS 'PlatformTwo' FROM edb.cases LEFT JOIN
edb.platform as numOne ON (numOne.rank = cases.platform) LEFT JOIN edb.platform as numTwo ON
(numTwo.rank = cases.highestPlatform) WHERE cases.index = ?
当我打印详细信息哈希图（即结果数据集）时，关键 PlatformTwo 完全不存在！
