Add full outer join

ktorm-core/src/main/kotlin/org/ktorm/dsl/QuerySource.kt

@@ -17,7 +17,11 @@
 package org.ktorm.dsl
 
 import org.ktorm.database.Database
-import org.ktorm.expression.*
+import org.ktorm.expression.BinaryExpression
+import org.ktorm.expression.BinaryExpressionType
+import org.ktorm.expression.JoinExpression
+import org.ktorm.expression.JoinType
+import org.ktorm.expression.QuerySourceExpression
 import org.ktorm.schema.BaseTable
 import org.ktorm.schema.BooleanSqlType
 import org.ktorm.schema.ColumnDeclaring
@@ -102,6 +106,20 @@ public fun QuerySource.rightJoin(right: BaseTable<*>, on: ColumnDeclaring<Boolea
     )
 }
 
+/**
+ * Perform a full outer join and return a new [QuerySource], translated to `full outer join` in SQL.
+ */
+public fun QuerySource.fullOuterJoin(right: BaseTable<*>, on: ColumnDeclaring<Boolean>? = null): QuerySource {
+    return this.copy(
+        expression = JoinExpression(
+            type = JoinType.FULL_OUTER_JOIN,
+            left = expression,
+            right = right.asExpression(),
+            condition = on?.asExpression()
+        )
+    )
+}
+
 /**
  * Return a new-created [Query] object, left joining all the reference tables, and selecting all columns of them.
  */

ktorm-core/src/main/kotlin/org/ktorm/expression/SqlExpressions.kt

@@ -412,7 +412,12 @@ public enum class JoinType(private val value: String) {
     /**
      * Right join, translated to the `right join` keyword in SQL.
      */
-    RIGHT_JOIN("right join");
+    RIGHT_JOIN("right join"),
+
+    /**
+     * Full outer join, translated to the `full outer join` keyword in SQL.
+     */
+    FULL_OUTER_JOIN("full outer join");
 
     override fun toString(): String {
         return value