`CASE...WHEN...THEN...END` Support

This is a bigger one, but, any plans on implementing CASE support?

https://modern-sql.com/feature/case

Looks like it is supported by: BigQuery, Db2, MariaDB, MySQL, Oracle DB, PostgreSQL, SQL Server, SQLite, so basically all major engines.

Syntax:

CASE
    WHEN condition1 THEN result1
    WHEN condition2 THEN result2
    WHEN conditionN THEN resultN
    ELSE result
END;

Probably would fit well as another method on ExpressionBuilder, ie:

db.selectFrom("table")
 .select(eb => {
  return eb.case(cb => {
   return cb.when("expression1", "expression2")
    .when("expression3", "expression4")
    .else("expression5")
  })
 })

[koskimas]

So we’d have orWhen, orWhenExists etc? That’s an option and it would take care of the case statement but it’s not reusable. I’m talking about a builder that could be used to build any expression. For example

select(eb => eb.bin('first_name', '=', 'Sami').as('is_sami'))

[koskimas]

If we go down the road of adding builders for all kinds of expressions, we need to really think it through and create a cohesive expression builder instead of adding different things ad-hoc. I’m also not sure we should do it at all. The code quickly becomes unreadable and it’s better just to write raw SQL and pay the small type-unsafety penalty.

Let’s think about the case builder for example. The syntax could indeed be something like this

eb.case(maybeExpr)
  .when(expr).then(expr)
  .when(expr).then(expr)
  .else(expr)

The expressions can be anything but let’s say they are simple comparison operations. We’d have two options:

1. Use raw SQL for each subexpression
2. Use the expression builder

Since we are on the road of full type-safety, we’d of course want to use the expression builder. We’d get something like this:

eb.case()
  .when(eb.bin('first_name', '=', 'foo')).then(eb.val(1))
  .when(eb.bin('first_name', 'in', ['bar', 'baz'])).then(eb.val(2))
  .else(eb.val(3))

In this example I used two non-existent expression builder methods bin and val. That’s not too bad yet, but compared to the raw SQL it’s quite a bit of boilerplate

sql<number>`
  case
    when first_name = 'foo' then 1
    when first_name in ('bar', 'baz') then 2
    else 3
  end
`