Snowflake - ALTER TABLE IF EXISTS and WHEN SYSTEM$STREAM_HAS_DATA()

src/sqlfluff/dialects/dialect_snowflake.py

@@ -347,6 +347,12 @@
         name="integer_literal",
         type="literal",
     ),
+    SystemFunctionName=RegexParser(
+        r"SYSTEM\$([A-z0-9]*)",
+        CodeSegment,
+        name="system_function_name",
+        type="system_function_name",
+    ),
     GroupByContentsGrammar=Delimited(
         OneOf(
             Ref("ColumnReferenceSegment"),
@@ -1464,6 +1470,7 @@ class AlterTableStatementSegment(ansi.AlterTableStatementSegment):
     match_grammar = Sequence(
         "ALTER",
         "TABLE",
+        Ref("IfExistsGrammar", optional=True),
         Ref("TableReferenceSegment"),
         OneOf(
             # Rename
@@ -2698,7 +2705,7 @@ class CreateTaskSegment(BaseSegment):
         Sequence(
             "WHEN",
             Indent,
-            Ref("ExpressionSegment"),
+            Ref("TaskExpressionSegment"),
             Dedent,
             optional=True,
         ),
@@ -2711,6 +2718,28 @@ class CreateTaskSegment(BaseSegment):
     )
 
 
+class TaskExpressionSegment(BaseSegment):
+    """Expressions for WHEN clause in TASK.
+
+    e.g. "SYSTEM$STREAM_HAS_DATA('MYSTREAM')"
+
+    """
+
+    type = "snowflake_task_expression_segment"
+    match_grammar = Sequence(
+        Delimited(
+            OneOf(
+                Ref("ExpressionSegment"),
+                Sequence(
+                    Ref("SystemFunctionName"),
+                    Bracketed(Ref("QuotedLiteralSegment")),
+                ),
+            ),
+            delimiter=OneOf(Ref("BooleanBinaryOperatorGrammar")),
+        )
+    )
+
+
 class CreateStatementSegment(BaseSegment):
     """A snowflake `CREATE` statement.
 

test/fixtures/dialects/snowflake/snowflake_alter_table_column.sql

@@ -65,3 +65,8 @@ alter table empl_info modify
 ALTER TABLE empl_info DROP COLUMN my_column;
 ALTER TABLE some_schema.empl_info DROP COLUMN my_column;
 ALTER TABLE my_table DROP COLUMN column_1, column_2, column_3;
+
+-- IF EXISTS
+ALTER TABLE IF EXISTS my_table ADD COLUMN my_column INTEGER;
+ALTER TABLE IF EXISTS empl_info DROP COLUMN my_column;
+ALTER TABLE IF EXISTS empl_info RENAME COLUMN old_col_name TO new_col_name;

test/fixtures/dialects/snowflake/snowflake_alter_table_column.yml

@@ -3,7 +3,7 @@
 # computed by SQLFluff when running the tests. Please run
 # `python test/generate_parse_fixture_yml.py`  to generate them after adding or
 # altering SQL files.
-_hash: 9c6169b3930295c33640ddc54adb221bf9df5ec14b9f39e401e0de5e53045a6d
+_hash: 64007c88f4ec9aaa64e70b57eb300056d0f40a5df3b4207b954ea20759a00361
 file:
 - statement:
     alter_table_statement:
@@ -544,3 +544,50 @@ file:
       - column_reference:
           identifier: column_3
 - statement_terminator: ;
+- statement:
+    alter_table_statement:
+    - keyword: ALTER
+    - keyword: TABLE
+    - keyword: IF
+    - keyword: EXISTS
+    - table_reference:
+        identifier: my_table
+    - alter_table_table_column_action:
+      - keyword: ADD
+      - keyword: COLUMN
+      - column_reference:
+          identifier: my_column
+      - data_type:
+          data_type_identifier: INTEGER
+- statement_terminator: ;
+- statement:
+    alter_table_statement:
+    - keyword: ALTER
+    - keyword: TABLE
+    - keyword: IF
+    - keyword: EXISTS
+    - table_reference:
+        identifier: empl_info
+    - alter_table_table_column_action:
+      - keyword: DROP
+      - keyword: COLUMN
+      - column_reference:
+          identifier: my_column
+- statement_terminator: ;
+- statement:
+    alter_table_statement:
+    - keyword: ALTER
+    - keyword: TABLE
+    - keyword: IF
+    - keyword: EXISTS
+    - table_reference:
+        identifier: empl_info
+    - alter_table_table_column_action:
+      - keyword: RENAME
+      - keyword: COLUMN
+      - column_reference:
+          identifier: old_col_name
+      - keyword: TO
+      - column_reference:
+          identifier: new_col_name
+- statement_terminator: ;

test/fixtures/dialects/snowflake/snowflake_create_task.sql

@@ -83,3 +83,13 @@ CREATE OR REPLACE TASK insert__agg
     SCHEDULE = 'USING CRON 15 7 2 * * UTC'
 AS
     CALL auto_device_insert();
+
+CREATE OR REPLACE TASK SCH.MY_TASK
+	WAREHOUSE = MY_WH
+	SCHEDULE = 'USING CRON 15 7 2 * * UTC'
+	USER_TASK_TIMEOUT_MS = 10800000
+WHEN
+    SYSTEM$STREAM_HAS_DATA('SCH.MY_STREAM')
+    AND 1=1
+AS
+    CALL SCH.MY_SPROC();

test/fixtures/dialects/snowflake/snowflake_create_task.yml

@@ -3,7 +3,7 @@
 # computed by SQLFluff when running the tests. Please run
 # `python test/generate_parse_fixture_yml.py`  to generate them after adding or
 # altering SQL files.
-_hash: ec9d42b15628824d95fd46d5486345a798957f38bdaa20137eb8bc76088bfecf
+_hash: 83f68e6b6527766cec861ad292d0e24643ee6e4d95f8009241cdd6b5c3d99459
 file:
 - statement:
     create_task_statement:
@@ -467,3 +467,52 @@ file:
               start_bracket: (
               end_bracket: )
 - statement_terminator: ;
+- statement:
+    create_task_statement:
+    - keyword: CREATE
+    - keyword: OR
+    - keyword: REPLACE
+    - keyword: TASK
+    - object_reference:
+      - identifier: SCH
+      - dot: .
+      - identifier: MY_TASK
+    - keyword: WAREHOUSE
+    - comparison_operator:
+        raw_comparison_operator: '='
+    - object_reference:
+        identifier: MY_WH
+    - keyword: SCHEDULE
+    - comparison_operator:
+        raw_comparison_operator: '='
+    - literal: "'USING CRON 15 7 2 * * UTC'"
+    - keyword: USER_TASK_TIMEOUT_MS
+    - comparison_operator:
+        raw_comparison_operator: '='
+    - literal: '10800000'
+    - keyword: WHEN
+    - snowflake_task_expression_segment:
+        system_function_name: SYSTEM$STREAM_HAS_DATA
+        bracketed:
+          start_bracket: (
+          literal: "'SCH.MY_STREAM'"
+          end_bracket: )
+        binary_operator: AND
+        expression:
+        - literal: '1'
+        - comparison_operator:
+            raw_comparison_operator: '='
+        - literal: '1'
+    - keyword: AS
+    - statement:
+        call_segment:
+          keyword: CALL
+          function:
+            function_name:
+              identifier: SCH
+              dot: .
+              function_name_identifier: MY_SPROC
+            bracketed:
+              start_bracket: (
+              end_bracket: )
+- statement_terminator: ;