Why does 'xxx is null' is generated in where condition when no null is actually compared?

[Teck Hooi Lim]

I wanted to create a Slick query similar to this SQL,

select lots.oid, lots.cat from lots

    left join bks on (lots.cat = bks.cat and bks.oid=100)

 where not exists (select 1 from bks_dates bd where

     lots.oid=bd.lots_oid and bd.bk_oid = bks.oid) and lots.e_oid=bk.e_oid

using,

val elQuery = slickEls
  .joinLeft(slickBks)
  .on((el, bk) => el.cat === bk.cat && bk.oid === bookingsOid && bk.etsOid === el.etsOid)
  .filter(x => x._2.map(_.etsOid === x._1.etsOid))

val finalQuery = for {
  a <- elQuery if !(slickBds.filter(bd => a._2.map(_.oid === bd.bksOid).isEmpty && a._1.oid === bd.elOid).exists)
} yield a

finalQuery.result.statements.foreach(x => println(s"xx => $x"))

What was printed was this,

select x2.`oid`, x2.`ets_oid`, x2.`cat`, x3.`ets_oid`, x3.`oid`, x3.`el_oid`, x3.`amount`, x3.`extras`, x3.`cat` from `lots` x2 

  left outer join `bks` x3 on ((x2.`cat` = x3.`cat`) and (x3.`oid` = 100)) 

    and (x3.`ets_oid` = x2.`ets_oid`) 

where (case when (x3.`ets_oid` is not null) then (x3.`ets_oid` = x2.`ets_oid`) else null end) 

and (not exists(select `bks_oid`, `oid`, `taken`, `el_oid`, `r_date`, `ets_oid` from `bk_dates` 

  where (x3.`ets_oid` is null) and (x2.`oid` = `el_oid`)))

Where did x3.`ets_oid` is null come from? How I do to fix a._2.map(_.oid === bd.bksOid).isEmpty to reflect the 

condition bd.bk_oid = bks.oid I wanted ? I have a feeling isEmpty is responsible for it. Thanks

This post is related to my post in http://stackoverflow.com/questions/36588765/slick-function-for-sql-exists/. 

Other supporting information for reference,

trait ForIssueDefinitions {

 // import driver.api._ // required but a Slick environment needed to be setup

 case class DBBk(oid: Option[Long],
   elOid: Option[Long],
   etsOid: Long,
   amount: Float,
   extras: Option[String],
   cat: Option[String])

 class Bks(tag: Tag) extends Table[DBBk](tag, "bks") {
   def oid = column[Option[Long]]("oid", O.AutoInc, O.PrimaryKey)
   def elOid = column[Option[Long]]("el_oid")
   def etsOid = column[Long]("ets_oid")
   def amount = column[Float]("amount")
   def extras = column[Option[String]]("extras")
   def cat = column[Option[String]]("cat")
   def * = (oid, elOid, etsOid, amount, extras, cat) <> ((DBBk.apply _).tupled, DBBk.unapply)
 }

 case class DBLot(oid: Option[Long],
   etsOid: Long,
   cat: Option[String])

 class Els(tag: Tag) extends Table[DBLot](tag, "lots") {
   def oid = column[Option[Long]]("oid", O.AutoInc, O.PrimaryKey)
   def etsOid = column[Long]("ets_oid")
   def cat = column[Option[String]]("cat")
   def * = (oid, etsOid, cat) <> ((DBLot.apply _).tupled, DBLot.unapply)
 }

 case class DBBd(oid: Option[Long],
   bksOid: Long,
   rDate: LocalDate,
   taken: Boolean,
   elOid: Option[Long],
   etsOid: Long
 )

 class Bds(tag: Tag) extends Table[DBBd](tag, "bk_dates") {
   def oid = column[Option[Long]]("oid", O.AutoInc, O.PrimaryKey)
   def bksOid = column[Long]("bks_oid")
   def rDate = column[LocalDate]("r_date")
   def taken = column[Boolean]("taken")
   def elOid = column[Option[Long]]("el_oid")
   def etsOid = column[Long]("ets_oid")
   def * = (oid, bksOid, rDate, taken, elOid, etsOid) <> ((DBBd.apply _).tupled, DBBd.unapply)
 }

 val slickBds = TableQuery[Bds]
 val slickEls = TableQuery[Els]
 val slickBks = TableQuery[Bks]
}