Add concatenation operator to Expr

[antoinecharbonneau]

In Postgres and many other database engine, you can concatenate columns using the || operator such as this example: first_name || ' ' || last_name. Some database such as MariaDB use the CONCAT function with multiple parameters such as CONCAT(first_name, ' ', last_name) to achieve the same thing.

This feature would allow, for instance, to use a single struct with QueryResult to aggregate data from multiple tables with similar purposes, such as tables that use Id and Label pattern, even if the label could be an aggregation of multiple columns like persons have a first and last name.

Provided a bit of guidance, I am open to spending a bit of my time to add the feature.

[Huliiiiii]

You can add corresponding functions and operators in sea query and add corresponding methods in ExprTrait.

[Expurple]

You can already achieve your goals with BinOper::Custom, Func::cust, or Expr::cust*.

@Huliiiiii's suggestions are for convenience. Sure, I'd welcome a contribution like that!

Here's an example of a recent PR that added new operators: SeaQL/sea-query#841. || operator can be added in exactly the same way. To add a CONCAT function, you need to add a Function::Concat variant and a Func::concat method.