java - 如何检查表中是否存在字符串？

Question

Here's the table (tblemployees)如何将使用文本文件的代码转换为将使用数据库(表)的代码。

int intcurrentLine = -1;
String[] strLineSplit = (sb.toString()).split("\\r?\\n"); // converts sb to string then splits it by line
int intNumElements = strLineSplit.length; // number of elements
while (intcurrentLine != (intNumElements - 1)) {
    intcurrentLine++;
    String[] strWords = (strLineSplit[intcurrentLine]).split(" ", 2); // splits the current line by the first instance(space)
    if (strEmpID.equals(strWords[0])) { // checks if the employee ID is available
        JOptionPane.showMessageDialog(null, "Welcome " + strWords[1] + ", you have successfully logged in.");
        strCheck = 1; // to confirm and go to time in and out process
        break;
    }
    if ((intcurrentLine + 1) == intNumElements) { // condition to state that ID cant be found from the employee list
        JOptionPane.showMessageDialog(null, "No such employee, please check the ID No. that you entered.");
    }
}

现在我想搜索包含员工编号的列。我如何把它放在一个条件下，我一直在寻找但无法找到明确的答案。他们只把如何搜索这样

String queryCheck = "SELECT * from messages WHERE EmpIDNo = 'COMSCI0001'";
ResultSet res = st.executeQuery(queryCheck);

然后我迷路了，如果员工没有，如何制定条件。不存在 某些事情会发生 否则某些事情会发生。我只是搞不清楚如何为此设定条件。

Answer 1

你可以这样做:

String queryCheck = "SELECT * FROM messages WHERE EmpIDNo = 'COMSCI0001' LIMIT 1";
ResultSet res = st.executeQuery(queryCheck);
boolean exists = res.next();

boolean 变量exists 将指示匹配记录是否存在。

请注意，我在 SQL 末尾添加了 LIMIT 1 作为优化，以避免获取比实际需要更多的数据。