fix(executor): handle subqueries in comparisons and per-row UPDATE expressions

- Add missing nkSubquery case in lowerExpr so scalar subqueries
  (e.g. budget > (SELECT AVG(budget) FROM projects)) are lowered
  correctly instead of being treated as NULL literals.
- Move UPDATE SET expression evaluation inside the row loop so
  column references (e.g. salary + 5000) are resolved against the
  current row, fixing type validation failures.
- Make irAdd/irSub/irMul/irDiv return integer strings when the
  result is a whole number, avoiding spurious float output for INT
  columns (e.g. MERGE UPDATE qty = 100 + 50 now yields 150).

These fix regressions introduced by switching NULL representation
from empty string to \N, which exposed the pre-existing bugs above.

Refs: issue #9, issue #10

src/barabadb/query/executor.nim

@@ -581,18 +581,34 @@ proc evalExpr*(expr: IRExpr, row: Table[string, string], ctx: ExecutionContext =
       if left == "true" or right == "true": return "true"
       return "false"
     of irAdd:
-      try: return $(parseFloat(left) + parseFloat(right))
+      try:
+        let sum = parseFloat(left) + parseFloat(right)
+        if sum == float(int(sum)):
+          return $int(sum)
+        return $sum
       except: return left & right
     of irSub:
-      try: return $(parseFloat(left) - parseFloat(right))
+      try:
+        let diff = parseFloat(left) - parseFloat(right)
+        if diff == float(int(diff)):
+          return $int(diff)
+        return $diff
       except: return "0"
     of irMul:
-      try: return $(parseFloat(left) * parseFloat(right))
+      try:
+        let prod = parseFloat(left) * parseFloat(right)
+        if prod == float(int(prod)):
+          return $int(prod)
+        return $prod
       except: return "0"
     of irDiv:
       try:
         let r = parseFloat(right)
-        if r != 0: return $(parseFloat(left) / r)
+        if r != 0:
+          let quot = parseFloat(left) / r
+          if quot == float(int(quot)):
+            return $int(quot)
+          return $quot
         return "0"
       except: return "0"
     of irMod:
@@ -1528,6 +1544,9 @@ proc lowerExpr*(node: Node): IRExpr =
       result.binRight = lowerExpr(node.inRight)
   of nkExists:
     result = IRExpr(kind: irekExists)
+  of nkSubquery:
+    result = IRExpr(kind: irekSubquery)
+    result.subqueryPlan = lowerSelect(node.subQuery)
   of nkStar:
     result = IRExpr(kind: irekStar)
   of nkWindowExpr:
@@ -3166,6 +3185,12 @@ proc executeQuery*(ctx: ExecutionContext, astNode: Node, params: seq[WireValue]
   of nkUpdate:
     if stmt.updSet.len == 0: return okResult()
     # Simple UPDATE: scan table, filter by WHERE, apply SET
+    # Scan and apply
+    let rows = execScan(ctx, stmt.updTarget)
+    var count = 0
+    var kvPairs: seq[(string, seq[byte])]
+    for row in rows:
+      # Compute sets for this row (expressions may reference columns)
       var sets = initTable[string, string]()
       for s in stmt.updSet:
         if s.kind == nkBinOp and s.binOp == bkAssign:
@@ -3175,14 +3200,8 @@ proc executeQuery*(ctx: ExecutionContext, astNode: Node, params: seq[WireValue]
                       elif s.binRight.kind == nkFloatLit: $s.binRight.floatVal
                       elif s.binRight.kind == nkBoolLit: $s.binRight.boolVal
                       elif s.binRight.kind == nkNullLit: "\\N"
-                    else: evalNodeToString(s.binRight)
+                      else: evalExpr(lowerExpr(s.binRight), row, ctx)
             sets[s.binLeft.identName] = val
-
-    # Scan and apply
-    let rows = execScan(ctx, stmt.updTarget)
-    var count = 0
-    var kvPairs: seq[(string, seq[byte])]
-    for row in rows:
       # Check WHERE
       if stmt.updWhere != nil and stmt.updWhere.whereExpr != nil:
         let whereExpr = lowerExpr(stmt.updWhere.whereExpr)

tests/test_all.nim

@@ -2997,7 +2997,7 @@ suite "MERGE Statement":
     check r.success
     check r.affectedRows == 1
     let verify = qexec.executeQuery(ctx, parse("SELECT * FROM inventory WHERE sku = 'SKU001'"))
-    check verify.rows[0]["qty"] == "150.0"
+    check verify.rows[0]["qty"] == "150"
 
   test "MERGE WHEN NOT MATCHED INSERT":
     let r = qexec.executeQuery(ctx, parse("""