Alias a table in Knex | 易学教程

问题

I have a SQL query that refers to the same table twice, and I need to alias the table to two separate aliases. I can't quite figure out how to compose this with Knex.

There's a 'Words' table and a 'Users' table. The Words table has two foreign keys, 'author_id' and 'winner_id', referencing the Users table's 'id' column.

Here's the SQL I'm trying to compose in Knex:

SELECT w.*, ua.name, uw.name FROM Words AS w
INNER JOIN Users AS ua ON w.author_id = ua.id 
LEFT JOIN Users AS uw ON w.winner_id = uw.id

I'm a little lost as to how to do this in Knex. My first attempt didn't involve aliasing, and so I got a 'table used more than once' error. When I tried to use the .as() method, knex complained that there was a missing .from() clause. Is the .as() method only used for aliasing subqueries, and I shouldn't expect it to be used to alias tables?

回答1:

I think I figured it out. In knex.js, say you specify a table like:

knex.select( '*' ).from( 'Users' )

Then you can just add the AS keyword within the quotes of the table name to alias it, like so:

knex.select( '*' ).from( 'Users AS u' )

..and you can do this for column names, too; so my original SQL would look like this in knex-land:

    knex.select( 'w.*', 'ua.name AS ua_name', 'uw.name AS uw_name' )
    .innerJoin( 'Users AS ua', 'author_id', 'ua.id' )
    .leftJoin( 'Users as uw', 'winner_id', 'uw.id' )

I guess I got confused by the presence of knex's .as() method, which (as far as I currently understand) is meant just for subqueries, not for aliasing tables or column names.

回答2:

Found this question when tried to figure out how to select all columns from all joined tables without overriding one another if column names are equals. This is how I did it, prepending each with its "tablename_":

const columnToText = (table, column) => `${table}.${column} as ${table}_${column}`;
const prepareColumns = async (table) => {
  const columnsInfo = await knex(table).columnInfo();
  return Object.keys(columnsInfo).map(column => columnToText(table, column));
};

const selectColumns = (await Promise.all([
  'joined_table1',
  'joined_table2',
  'main_table',
].map(prepareColumns)))
  .reduce((acc, item) => ([...acc, ...item]), []);

const data = await knex('main_table')
  .leftJoin('joined_table1', 'main_table.joined_table1_id', 'joined_table1.id')
  .leftJoin('joined_table2', 'main_table.joined_table1_id', 'joined_table2.id')
  .select(...selectColumns);

回答3:

Aliasing or the .as() method is not only used for subqueries, it can also be used in aliasing column names and tables. In your case you need to use alias to your table because you used the table twice. In your first attempt, It got an error because your SQL got confused. By putting an aliases on the table that is used twice, you gave it an identification that ua.Users table is now different to uw.Users table. Hope it helped you.

来源：https://stackoverflow.com/questions/28357228/alias-a-table-in-knex

标签

sql

left-join

inner-join

knex.js