added isnotnull condition support

docs/user/ppl/functions/condition.md

@@ -7,6 +7,8 @@ PPL conditional functions enable global filtering of query results based on spec
 
 Returns `TRUE` if the field is `NULL`, `FALSE` otherwise.
 
+The `field IS NULL` predicate syntax is also supported as a synonym.
+
 The `isnull()` function is commonly used:
 - In `eval` expressions to create conditional fields.
 - With the `if()` function to provide default values.
@@ -69,6 +71,14 @@ source=accounts
 | where isnull(employer)
 | fields account_number, firstname, employer
 ```
+
+The `IS NULL` predicate syntax can be used as an equivalent alternative:
+
+```ppl
+source=accounts
+| where employer IS NULL
+| fields account_number, firstname, employer
+```
 
 The query returns the following results:
 
@@ -87,6 +97,8 @@ fetched rows / total rows = 1/1
 
 Returns `TRUE` if the field is NOT `NULL`, `FALSE` otherwise.
 
+The `field IS NOT NULL` predicate syntax is also supported as a synonym.
+
 The `isnotnull()` function is commonly used:
 - In `eval` expressions to create Boolean flags.
 - In `where` clauses to filter out null values.
@@ -141,6 +153,27 @@ fetched rows / total rows = 1/1
 | 18             | null     |
 +----------------+----------+
 ```
+
+The `IS NOT NULL` predicate syntax is equivalent to `isnotnull()`:
+
+```ppl
+source=accounts
+| where employer IS NOT NULL
+| fields account_number, employer
+```
+
+The query returns the following results:
+
+```text
+fetched rows / total rows = 3/3
++----------------+----------+
+| account_number | employer |
+|----------------+----------|
+| 1              | Pyrami   |
+| 6              | Netagy   |
+| 13             | Quility  |
++----------------+----------+
+```
 
 The following example demonstrates using `isnotnull` with the `if` function to create validation messages:
 

integ-test/src/test/java/org/opensearch/sql/ppl/WhereCommandIT.java

@@ -193,6 +193,27 @@ public void testIsNotNullFunction() throws IOException {
     verifyDataRows(result, rows("Amber JOHnny"));
   }
 
+  @Test
+  public void testIsNullPredicate() throws IOException {
+    JSONObject result =
+        executeQuery(
+            String.format(
+                "source=%s | where age IS NULL | fields firstname",
+                TEST_INDEX_BANK_WITH_NULL_VALUES));
+    verifyDataRows(result, rows("Virginia"));
+  }
+
+  @Test
+  public void testIsNotNullPredicate() throws IOException {
+    JSONObject result =
+        executeQuery(
+            String.format(
+                "source=%s | where age IS NOT NULL and like(firstname, 'Ambe_%%') | fields"
+                    + " firstname",
+                TEST_INDEX_BANK_WITH_NULL_VALUES));
+    verifyDataRows(result, rows("Amber JOHnny"));
+  }
+
   @Test
   public void testWhereWithMetadataFields() throws IOException {
     JSONObject result =

ppl/src/main/antlr/OpenSearchPPLLexer.g4

@@ -187,7 +187,9 @@ PATH:                               'PATH';
 CASE:                               'CASE';
 ELSE:                               'ELSE';
 IN:                                 'IN';
+IS:                                 'IS';
 EXISTS:                             'EXISTS';
+NULL:                               'NULL';
 
 // Geo IP eval function
 GEOIP:                              'GEOIP';

ppl/src/main/antlr/OpenSearchPPLParser.g4

@@ -911,6 +911,11 @@ expression
    | left = expression comparisonOperator right = expression    # compareExpr
    | expression NOT? IN LT_PRTHS valueList RT_PRTHS             # inExpr
    | expression NOT? BETWEEN expression AND expression          # between
+   | expression IS nullNotnull                                  # isNullPredicate
+   ;
+
+nullNotnull
+   : NOT? NULL
    ;
 
 
@@ -1594,6 +1599,8 @@ wildcard
 keywordsCanBeId
    : searchableKeyWord
    | IN
+   | IS
+   | NULL
    ;
 
 searchableKeyWord

ppl/src/main/java/org/opensearch/sql/ppl/parser/AstExpressionBuilder.java

@@ -249,6 +249,15 @@ public UnresolvedExpression visitInExpr(InExprContext ctx) {
     return ctx.NOT() != null ? new Not(expr) : expr;
   }
 
+  @Override
+  public UnresolvedExpression visitIsNullPredicate(OpenSearchPPLParser.IsNullPredicateContext ctx) {
+    return new Function(
+        ctx.nullNotnull().NOT() == null
+            ? IS_NULL.getName().getFunctionName()
+            : IS_NOT_NULL.getName().getFunctionName(),
+        Arrays.asList(visit(ctx.expression())));
+  }
+
   /** Value Expression. */
   @Override
   public UnresolvedExpression visitBinaryArithmetic(BinaryArithmeticContext ctx) {

ppl/src/test/java/org/opensearch/sql/ppl/parser/AstExpressionBuilderTest.java

@@ -299,6 +299,24 @@ public void testBooleanIsNotNullFunction() {
         filter(relation("t"), function("is not null", field("a"))));
   }
 
+  @Test
+  public void testIsNullPredicate() {
+    assertEqual(
+        "source=t | where a is null", filter(relation("t"), function("is null", field("a"))));
+    assertEqual(
+        "source=t | where a IS NULL", filter(relation("t"), function("is null", field("a"))));
+  }
+
+  @Test
+  public void testIsNotNullPredicate() {
+    assertEqual(
+        "source=t | where a is not null",
+        filter(relation("t"), function("is not null", field("a"))));
+    assertEqual(
+        "source=t | where a IS NOT NULL",
+        filter(relation("t"), function("is not null", field("a"))));
+  }
+
   /** Todo. search operator should not include functionCall, need to change antlr. */
   @Ignore("search operator should not include functionCall, need to change antlr")
   public void testEvalExpr() {