From 5eea5d58eb1455ece56cefe1edad34cc72687bda Mon Sep 17 00:00:00 2001
From: Kyrylo Simonov <xi@resolvent.net>
Date: Fri, 21 Apr 2023 08:54:34 -0500
Subject: [PATCH] Improved handling of LIMIT clause in SQL Server

---
 docs/src/test/clauses.md | 54 +++++++++++++++++++++++++++++++++++++++-
 src/dialects.jl          |  6 ++++-
 src/serialize.jl         | 25 ++++++++++++++++---
 3 files changed, 79 insertions(+), 6 deletions(-)

diff --git a/docs/src/test/clauses.md b/docs/src/test/clauses.md
index 76fc9572..9781b649 100644
--- a/docs/src/test/clauses.md
+++ b/docs/src/test/clauses.md
@@ -768,7 +768,6 @@ Non-standard MySQL and SQLite syntax is supported.
     LIMIT 10
     =#
 
-
 Both limit (the number of rows) and offset (number of rows to skip) can
 be specified.
 
@@ -865,6 +864,59 @@ It is possible to specify the limit with ties.
     FETCH FIRST 10 ROWS WITH TIES
     =#
 
+SQL Server automatically replaces `FETCH` clause with `TOP` when possible.
+
+    c = FROM(:person) |>
+        ORDER(:year_of_birth) |>
+        LIMIT(10) |>
+        SELECT(:person_id)
+
+    print(render(c, dialect = :sqlserver))
+    #=>
+    SELECT TOP 10 [person_id]
+    FROM [person]
+    ORDER BY [year_of_birth]
+    =#
+
+    c = FROM(:person) |>
+        ORDER(:year_of_birth) |>
+        LIMIT(100, 10) |>
+        SELECT(:person_id)
+
+    print(render(c, dialect = :sqlserver))
+    #=>
+    SELECT [person_id]
+    FROM [person]
+    ORDER BY [year_of_birth]
+    OFFSET 100 ROWS
+    FETCH NEXT 10 ROWS ONLY
+    =#
+
+SQL Server prohibits `ORDER BY` without limiting in a nested query, so FunSQL
+automatically adds `OFFSET 0` clause to the query.
+
+    c = FROM(:person) |>
+        ORDER(:year_of_birth) |>
+        SELECT(:person_id, :gender_concept_id) |>
+        AS(:person) |>
+        FROM() |>
+        WHERE(FUN("=", :gender_concept_id, 8507)) |>
+        SELECT(:person_id)
+
+    print(render(c, dialect = :sqlserver))
+    #=>
+    SELECT [person_id]
+    FROM (
+      SELECT
+        [person_id],
+        [gender_concept_id]
+      FROM [person]
+      ORDER BY [year_of_birth]
+      OFFSET 0 ROWS
+    ) AS [person]
+    WHERE ([gender_concept_id] = 8507)
+    =#
+
 
 ## `JOIN` Clause
 
diff --git a/src/dialects.jl b/src/dialects.jl
index 64cbb23c..e370c618 100644
--- a/src/dialects.jl
+++ b/src/dialects.jl
@@ -28,6 +28,7 @@ module LIMIT_STYLE
     ANSI
     MYSQL
     SQLITE
+    SQLSERVER
 end
 
 Base.convert(::Type{LimitStyle}, s::Symbol) =
@@ -37,8 +38,10 @@ Base.convert(::Type{LimitStyle}, s::Symbol) =
         MYSQL :
     s in (:sqlite, :SQLITE) ?
         SQLITE :
+    s in (:sqlserver, :SQLSERVER) ?
+        SQLSERVER :
     throw(DomainError(QuoteNode(s),
-                      "expected :ansi, :mysql, or :sqlite"))
+                      "expected :ansi, :mysql, :sqlite, or :sqlserver"))
 
 end
 
@@ -154,6 +157,7 @@ const sqlserver_dialect =
                has_boolean_literals = false,
                has_recursive_annotation = false,
                identifier_quotes = ('[', ']'),
+               limit_style = LIMIT_STYLE.SQLSERVER,
                values_column_prefix = nothing,
                variable_prefix = '?',
                variable_style = VARIABLE_STYLE.POSITIONAL)
diff --git a/src/serialize.jl b/src/serialize.jl
index f3e9d7a0..ce31607b 100644
--- a/src/serialize.jl
+++ b/src/serialize.jl
@@ -746,11 +746,25 @@ function serialize!(c::SelectClause, ctx)
         newline(ctx)
     end
     ctx.nested = true
-    print(ctx, "SELECT")
+    over = c.over
+    limit = nothing
+    with_ties = false
+    offset_0_rows = false
     top = c.top
     if top !== nothing
-        print(ctx, " TOP ", top.limit)
-        if top.with_ties
+        limit = top.limit
+        with_ties = top.with_ties
+    elseif ctx.dialect.limit_style === LIMIT_STYLE.SQLSERVER
+        if @dissect(over, limit_over |> LIMIT(offset = nothing, limit = limit, with_ties = with_ties))
+            over = limit_over
+        elseif nested && @dissect(over, ORDER())
+            offset_0_rows = true
+        end
+    end
+    print(ctx, "SELECT")
+    if limit !== nothing
+        print(ctx, " TOP ", limit)
+        if with_ties
             print(ctx, " WITH TIES")
         end
     end
@@ -758,10 +772,13 @@ function serialize!(c::SelectClause, ctx)
         print(ctx, " DISTINCT")
     end
     serialize_lines!(c.args, ctx)
-    over = c.over
     if over !== nothing
         serialize!(over, ctx)
     end
+    if offset_0_rows
+        newline(ctx)
+        print(ctx, "OFFSET 0 ROWS")
+    end
     ctx.nested = nested
     if nested
         ctx.level -= 1