How can I identify columns when SELECTing from multiple tables with JDBC?

问题

I have two tables that I join on the id-column, they look like:

+-------+
| users |
+----+--+---+
| id | name |
+----+------+

+-------+
| posts |
+-------+------+---------+
| id | user_id | message |
+----+---------+---------+

And now I want to select all posts and include the username, with:

SELECT * FROM posts, users WHERE user_id = users.id

And then I try to get the values with:

ResultSet rs = // SQL
if(rs.next()) {
    rs.getInt("posts.id");
    ...
}

But I get SQLException when executing rs.getInt("posts.id") :

java.sql.SQLException: Column 'posts.id' not found.

How can I get the values from the SQL-query above using JDBC, and JavaDB/Derby as database?

How can I distinguish between the id column in the users and posts table when retrieving values with ResultSet?

回答1:

You're attempting to retrieve the id value, but you're using "posts.id" to reference it. Don't

All you need is the column name or alias, not the table name as well:

ResultSet rs = // SQL
if(rs.next()) {
  rs.getInt("id");
  ...
}

It would've worked if your column name itself was "posts.id", but I recommend using underscore (_) instead of a period should you choose to update the table.

But I have an id column in both tables, how do i distinguish between them?

You need to specify a column alias:

SELECT p.id AS post_id,
       p.name,
       u.id AS users_id, 
       p.user_id, --redundant due to relationship, omit if possible
       u.message
  FROM POSTS p
  JOIN USERS u ON u.id = p.user_id

...and reference that column alias in the Java code:

ResultSet rs = // SQL
if(rs.next()) {
  rs.getInt("post_id");
  ...
}

回答2:

Solution 1 : use alias

select u.id as uid, u.name, p.id as post_id, p.user_id, p.message from
users u inner join posts p on u.id=p.user_id

Solution 2: remove the duplicate user.id since you already have it in the posts table

select p.user_id, u.name, p.id as post_id, p.message from
users u inner join posts p on u.id=p.user_id

回答3:

Alias the column names

SELECT
    posts.id posts_id,
    name name,
    id id,
    user_id,
    message
FROM posts 
INNER JOIN users 
    ON posts.user_id = users.id

depending on your sql flavour, that alias may need to be

posts.id as posts_id

回答4:

How can I get the values from the SQL-query above using JDBC, and JavaDB/Derby as database?

To get values, you should use the ResultSet's column name. The column name used in the ResultSet will be original column name or the alias.

If your ResultSet has columns with the same name or alias, the getInt() will return the first match.

With your select ...

SELECT * FROM posts, users WHERE user_id = users.id

...the rs.getInt() will return only the posts.id since it will the first column.

How can I distinguish between the id column in the users and posts table when retrieving values with ResultSet?

You can use a unique alias for every column or you can use the ResultSetMetaData

Solution 1: Use SQL alias

SQL

SELECT p.id AS post_id,
    p.name AS post_name,
    u.id AS user_id, 
    u.message AS user_message
  FROM POSTS p
  JOIN USERS u
    ON u.id = p.user_id

Java

ResultSet rs = // SQL
if(rs.next()) {
  rs.getInt("post_id");
  rs.getInt("user_id");
}

Solution 2: Use ResultSetMetaData

SQL

SELECT p.id
    p.name
    u.id
    u.message
  FROM POSTS p
  JOIN USERS u
    ON u.id = p.user_id

Java

Integer userID;
Integer postID;

ResultSetMetaData rsMeta = rs.getMetaData();
if (rs.next()) {
  for (int i = 1; i <= rsMeta.getColumnCount(); i++) {
    String table = rsMeta.getTableName(i);
    String col = rsMeta.getColumnName(i);

    if(table.equals("users") && col.equals("id")){
      userID = rs.getInt(i);
      continue;
    }

    if(table.equals("posts") && col.equals("id")) {
      userID = rs.getInt(i);
      continue;
    }
  }
}

回答5:

you can simplify these further by getting data as per the column index instead of getting it by column name..!! what ever data we retrieve from DB is stored in result set in the column order as per the select statement so instead of getting data as per the column name prefer fetching data as per the column index in result set..

ex:

String query = " select age,num from table_abc";
ResultSet rs= statement.executeQuery(query);

while(rs.next){
   int var1= rs.getInt(1);
   int var2= rs.getInt(2);
}

these will help you do away with complexity and confusion created due to similar column names in your database tables..

Hope these helps...

All the best..!!

来源：https://stackoverflow.com/questions/2970273/how-can-i-identify-columns-when-selecting-from-multiple-tables-with-jdbc

标签

sql