Question

我收到了这个问题：

SELECT * FROM Subject

使用此WHERE子句：

WHERE Tag like '%something%'

此查询适用于SQL Manager，但JDBC会因此异常而崩溃：

Incorrect syntax near the keyword 'WHERE'.

代码是：

String query = "SELECT * FROM Subject";
if (tags != null && tags.length>0) {
    for(int i = 0;i<tags.length;i++){
       query = query + " WHERE Tag like '%" + tags[i] + "%'";
    }
}

等等。

为什么这不正确？

Answer 1

当然它会崩溃，因为最终查询不正确，请考虑您有这条信息：

String tags[] = {"aa", "bb", "cc"};
String query = "SELECT * FROM Subject";

for (int i = 0; i < tags.length; i++) {
    query = query + " WHERE Tag like '%" + tags[i] + "%'";
}
System.out.println(query);

这应该返回：

SELECT * FROM Subject WHERE Tag like '%aa%' WHERE Tag like '%bb%' WHERE Tag like '%cc%'
//---------------------^^--------------------^^--------------------^^

这不正确的语法。

要避免此问题，您必须使用：

String query = "SELECT * FROM Subject WHERE ";
String or = "";
for (int i = 0; i < tags.length; i++) {
    query += or +" Tag like '%" + tags[i] + "%'";
    or = " OR ";
}

这可以告诉你：

SELECT * FROM Subject WHERE  Tag like '%aa%' OR  Tag like '%bb%' OR  Tag like '%cc%'

注意

这仍然不完美，以避免任何语法错误或SQL注入我建议使用PreparedStatement，例如：

for (int i = 0; i < tags.length; i++) { //query += or + " Tag like '%" + tags[i] + "%'"; query += or + " Tag like ?"; or = " OR "; } //query = SELECT * FROM Subject WHERE Tag like ? OR Tag like ? OR Tag like ? Connection connection = null; try (PreparedStatement stm = connection.prepareStatement(query)) { for (int i = 1; i <= tags.length; i++) { stm.setString(i, "%" + tags[i-1] + "%");//set values to your query } ResultSet rs = stm.executeQuery();//execute your query while(rs.next()){ //get your results } }

Answer 2

你正在循环＃34; WHERE＆＃34;。添加每个新标签。

    String[] tags = new String[2];
    tags[0] = "bob";
    tags[1] = "hank";

    String query = "SELECT * FROM Subject";

    if (tags != null && tags.length>0) {
        for(int i = 0;i<tags.length;i++){
            query = query + " WHERE Tag like '%" + tags[i] + "%'";
        }
    }

    System.out.println(query);

这将返回

SELECT * FROM Subject WHERE Tag like '%bob%' WHERE Tag like '%hank%'

Answer 3

    String query = "SELECT * FROM Subject";

    if (tags != null && tags.length > 0) {
        query += " WHERE "; // do this once

        for(int i = 0;i < tags.length; i++){
            query += " Tag like '%" + tags[i] + "%'";
            query += " OR ";
        }

        query = query.substring(0, query.lenght() - 3); // Remove last "OR"
    }

Answer 4

发现问题所在。我没有在每个循环的回合开始时重置查询。实际上每次都添加了一个WHERE子句。现在它工作正常。

JDBC关键字WHERE附近的语法不正确

4 个答案: