我正在尝试制作简单的java代码,以检查MySQL DB中是否存在表和/或列。我应该使用Java代码进行检查还是创建SQL查询字符串并执行它来进行检查?
编辑 -
@ aleroot -
我尝试使用您的代码,如下所示。当我运行下面的代码时,我没有看到任何表或列。我只看到这个 -
Driver Loaded.
Got Connection.
我的数据库有很多数据库,表和列。我不知道为什么这个程序正常工作。
import java.sql.Connection;
import java.sql.DatabaseMetaData;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.Statement;
public class Tester {
static Connection conn;
static Statement st;
public static void main(String[] args) throws Exception {
try {
// Step 1: Load the JDBC driver.
System.out.println("Driver Loaded.");
// Step 2: Establish the connection to the database.
String url = "jdbc:mysql://localhost:3306/";
conn = DriverManager.getConnection(url, "cowboy", "123456");
System.out.println("Got Connection.");
st = conn.createStatement();
} catch (Exception e) {
System.err.println("Got an exception! ");
e.printStackTrace();
System.exit(0);
}
DatabaseMetaData md2 = conn.getMetaData();
ResultSet rsTables = md2.getColumns(null, null, "customers", "name");
if (rsTables.next()) {
System.out.println("Exists !");
}
}
}
答案 0 :(得分:30)
要检查表格是否存在,您可以这样使用DatabaseMetaData:
DatabaseMetaData md = connection.getMetaData();
ResultSet rs = md.getTables(null, null, "table_name", null);
if (rs.next()) {
//Table Exist
}
要检查列是否存在,您可以以类似的方式使用它:
DatabaseMetaData md = connection.getMetaData();
ResultSet rs = md.getColumns(null, null, "table_name", "column_name");
if (rs.next()) {
//Column in table exist
}
答案 1 :(得分:1)
SELECT * FROM INFORMATION_SCHEMA.COLUMNS
WHERE TABLE_SCHEMA='table_schema'
AND TABLE_NAME='table_name'
AND COLUMN_NAME='column_name'
如果此sql什么都不返回 - 列不存在。您可以在java端执行此查询,但这取决于您使用的内容(JPA,JDBC,JDBCTemplate)。
答案 2 :(得分:0)
知道您可以使用以下SQL语句(在MySQL中)从表中实际选择一列:
show columns from TABLE_NAME where field = 'FIELD_NAME';
您可以执行以下操作:
...
PreparedStatement preparedStatement =
connection.prepareStatement(
"show columns from [TABLE_NAME] where field = ?");
preparedStatement.setString(1, columName);
ResultSet resultSet = preparedStatement.executeQuery();
if (resultSet.next()) {
System.out.println("column exists!");
} else {
System.out.println("column doesn't exists!");
}
...
答案 3 :(得分:0)
在Apache Derby(SQL)
中E.g。检查列是否存在:
SELECT ss.SCHEMANAME, st.TABLENAME, sc.COLUMNNAME FROM SYS.SYSSCHEMAS ss
INNER JOIN SYS.SYSTABLES st ON st.SCHEMAID = ss.SCHEMAID AND st.TABLENAME = <YOUR TABLE NAME>
INNER JOIN SYS.SYSCOLUMNS sc ON sc.REFERENCEID = st.TABLEID AND sc.COLUMNNAME = <YOUR COLUMN NAME>
WHERE ss.SCHEMANAME = <YOUR SCHEMA>