feat(knex): allow changing FROM clause using QueryBuilder

closes #3374

docs/docs/query-builder.md

@@ -296,6 +296,34 @@ const count = await qb.getCount();
 
 This will also remove any existing limit and offset from the query (the QB will be cloned under the hood, so calling `getCount()` does not mutate the original QB state).
 
+## Overriding FROM clause
+
+You can specify the table used in the `FROM` clause, replacing the current table name if one has already been specified. This is typically used to specify a sub-query expression in SQL. 
+
+```ts
+const qb = em.createQueryBuilder(Book2);
+qb.select('*').from(Author2).where({ id: { $gt: 2 } });
+// select `e0`.* from `author2` as `e0` where `e0`.`id` > 2;
+```
+
+You can also use sub-queries in the `FROM` like this:
+
+```ts
+const qb1 = em.createQueryBuilder(Book2).where({ id: { $lte: new Date() } }).orderBy({ id: 'DESC' }).limit(10);
+const qb2 = em.createQueryBuilder(Book2).from(qb.clone())
+qb.select('*').orderBy({ id: 'ASC' });
+// select `e1`.* from (select `e0`.* from `book2` as `e0` where `e0`.`id` <= ? order by `e0`.`id` desc limit ?) as `e1` order by `e1`.`id`;
+```
+
+To set up an alias to refer to a table in a `SELECT` statement, pass the second argument as follows:
+
+```ts
+const qb1 = em.createQueryBuilder(Book2, 'b1').where({ id: { $lte: new Date() } }).orderBy({ id: 'DESC' }).limit(10);
+const qb2 = em.createQueryBuilder(Book2, 'b2').from(qb.clone())
+qb.select('*').orderBy({ id: 'ASC' });
+// select `b2`.* from (select `b1`.* from `book2` as `b1` where `b1`.`id` <= ? order by `b1`.`id` desc limit ?) as `b2` order by `b2`.`id`;
+```
+
 ## Using sub-queries
 
 You can filter using sub-queries in where conditions:

packages/knex/src/query/Alias.ts

@@ -0,0 +1,15 @@
+import type { Knex } from 'knex';
+import type { EntityMetadata } from '@mikro-orm/core';
+
+export class Alias {
+
+  aliasName!: string;
+  entityName!: string;
+  metadata?: EntityMetadata;
+  subQuery?: Knex.QueryBuilder;
+
+  constructor(params: Alias) {
+    Object.assign(this, params);
+  }
+
+}