Add IF NOT EXISTS support when creating tables

README.md

@@ -107,6 +107,18 @@ Clickhouse.connection.exists_table("events")
 => true
 ```
 
+You can even check table existance upon creation.
+
+```ruby
+Clickhouse.connection.create_table("events") do |t|
+  t.if_not_exists
+  t.fixed_string :id, 16
+  t.uint16       :year
+  t.date         :date
+  t.date_time    :time
+  t.string       :event
+```
+
 Insert data.
 
 ```ruby

lib/clickhouse/connection/query/table.rb

@@ -5,6 +5,7 @@ class Table
 
         def initialize(name)
           @name = name
+          @if_not_exists = false
           @columns = []
           yield self
         end
@@ -13,12 +14,20 @@ def engine(value)
           @engine = value
         end
 
+        def if_not_exists(value = true)
+          @if_not_exists = value
+        end
+
         def to_sql
           raise Clickhouse::InvalidQueryError, "Missing table engine" unless @engine
           length = @columns.collect{|x| x[0].to_s.size}.max
 
           sql = []
-          sql << "CREATE TABLE #{@name} ("
+          if @if_not_exists
+            sql << "CREATE TABLE IF NOT EXISTS #{@name} ("
+          else
+            sql << "CREATE TABLE #{@name} ("
+          end
 
           @columns.each_with_index do |(name, type), index|
             sql << "  #{name.ljust(length, " ")} #{type}#{"," unless index == @columns.size - 1}"

test/unit/connection/query/test_table.rb

@@ -17,16 +17,43 @@ class TestTable < MiniTest::Test
               t.engine       "MergeTree(date, 8192)"
             end
 
-            sql = <<-SQL
-CREATE TABLE logs_test (
-  id     UInt8,
-  price  Float32,
-  name   String,
-  date   Date,
-  time   DateTime,
-  hex_id FixedString(8)
-)
-ENGINE = MergeTree(date, 8192)
+            sql = <<~SQL
+              CREATE TABLE logs_test (
+                id     UInt8,
+                price  Float32,
+                name   String,
+                date   Date,
+                time   DateTime,
+                hex_id FixedString(8)
+              )
+              ENGINE = MergeTree(date, 8192)
+            SQL
+
+            assert_equal sql.strip, table.to_sql.strip
+          end
+
+          it "generates a 'CREATE TABLE IF NOT EXISTS' statement" do
+            table = Clickhouse::Connection::Query::Table.new("logs_test") do |t|
+              t.if_not_exists
+              t.uint8         :id
+              t.float32       :price
+              t.string        :name
+              t.date          :date
+              t.date_time     :time
+              t.fixed_string  :hex_id, 8
+              t.engine        "MergeTree(date, 8192)"
+            end
+
+            sql = <<~SQL
+              CREATE TABLE IF NOT EXISTS logs_test (
+                id     UInt8,
+                price  Float32,
+                name   String,
+                date   Date,
+                time   DateTime,
+                hex_id FixedString(8)
+              )
+              ENGINE = MergeTree(date, 8192)
             SQL
 
             assert_equal sql.strip, table.to_sql.strip