Merge pull request #2920 from VadimPE/CLICKHOUSE-3819

CLICKHOUSE-3819 add CASE without ELSE

dbms/src/Parsers/ParserCase.cpp

@@ -50,12 +50,18 @@ bool ParserCase::parseImpl(Pos & pos, ASTPtr & node, Expected & expected)
         if (!has_branch)
             return false;
 
-        if (!s_else.ignore(pos, expected))
-            return false;
-
         ASTPtr expr_else;
-        if (!p_expr.parse(pos, expr_else, expected))
-            return false;
+        if (s_else.ignore(pos, expected))
+        {
+            if (!p_expr.parse(pos, expr_else, expected))
+                return false;
+        }
+        else
+        {
+            Field field_with_null;
+            ASTLiteral null_literal(field_with_null);
+            expr_else = std::make_shared<ASTLiteral>(null_literal);
+        }
         args.push_back(expr_else);
 
         if (!s_end.ignore(pos, expected))

dbms/tests/queries/0_stateless/00688_case_without_else.reference

@@ -0,0 +1,4 @@
+0
+\N
+0
+\N

dbms/tests/queries/0_stateless/00688_case_without_else.sql

@@ -0,0 +1,9 @@
+DROP TABLE IF EXISTS test.test;
+
+CREATE TABLE test.test (a UInt8) ENGINE = Memory;
+
+INSERT INTO test.test VALUES (1), (2), (1), (3);
+
+SELECT CASE WHEN a=1 THEN 0 END FROM test.test;
+
+DROP TABLE test.test;