types: fix CASE WHEN wrong result when prepare value is NULL

pkg/planner/core/casetest/BUILD.bazel

@@ -12,7 +12,7 @@ go_test(
     ],
     data = glob(["testdata/**"]),
     flaky = True,
-    shard_count = 30,
+    shard_count = 31,
     deps = [
         "//pkg/domain",
         "//pkg/domain/infosync",

pkg/planner/core/casetest/integration_test.go

@@ -536,3 +536,29 @@ func TestIssue56915(t *testing.T) {
 		"└─TableRowIDScan(Probe) 1.00 cop[tikv] table:t keep order:false, stats:partial[j:unInitialized]",
 	))
 }
+
+func TestCaseWhenPreparedStatementBug(t *testing.T) {
+	store, _ := testkit.CreateMockStoreAndDomain(t)
+	tk := testkit.NewTestKit(t, store)
+
+	tk.MustExec("use test")
+	tk.MustExec("CREATE TABLE t0(c0 DECIMAL);")
+	tk.MustExec("INSERT INTO t0 VALUES (0);")
+
+	normalResult := tk.MustQuery("SELECT * FROM t0 WHERE CAST((CASE t0.c0 WHEN t0.c0 THEN CAST(t0.c0 AS TIME) ELSE NULL END ) AS DATE);")
+	t.Logf("Normal query result: %v", normalResult.Rows())
+
+	caseResult := tk.MustQuery("SELECT CAST((CASE t0.c0 WHEN t0.c0 THEN CAST(t0.c0 AS TIME) ELSE NULL END ) AS DATE) FROM t0;")
+	t.Logf("Normal CASE result: %v", caseResult.Rows())
+
+	tk.MustExec("SET @b = NULL;")
+	tk.MustExec("PREPARE prepare_query FROM 'SELECT * FROM t0 WHERE CAST((CASE t0.c0 WHEN t0.c0 THEN CAST(t0.c0 AS TIME) ELSE ? END ) AS DATE)';")
+	preparedResult := tk.MustQuery("EXECUTE prepare_query USING @b;")
+	t.Logf("Prepared statement result: %v", preparedResult.Rows())
+
+	tk.MustExec("PREPARE prepare_case FROM 'SELECT CAST((CASE t0.c0 WHEN t0.c0 THEN CAST(t0.c0 AS TIME) ELSE ? END ) AS DATE) FROM t0';")
+	preparedCaseResult := tk.MustQuery("EXECUTE prepare_case USING @b;")
+	t.Logf("Prepared CASE result: %v", preparedCaseResult.Rows())
+
+	require.Equal(t, len(normalResult.Rows()), len(preparedResult.Rows()), "Results should have the same number of rows")
+}

pkg/types/field_type.go

@@ -194,11 +194,14 @@ func InferParamTypeFromDatum(d *Datum, tp *FieldType) {
 func InferParamTypeFromUnderlyingValue(value any, tp *FieldType) {
 	switch value.(type) {
 	case nil:
-		tp.SetType(mysql.TypeVarString)
-		tp.SetFlen(UnspecifiedLength)
-		tp.SetDecimal(UnspecifiedLength)
-		// Also set the `charset` and `collation` for it, because some function (e.g. `json_object`) will return error
-		// if the argument collation is `binary`.
+		// For NULL parameters, use TypeNull to ensure consistent behavior with literal NULL values
+		// in control flow functions like CASE WHEN. Previously using TypeVarString caused incorrect
+		// type inference in prepared statements (issue #62564).
+		tp.SetType(mysql.TypeNull)
+		tp.SetFlen(0)
+		tp.SetDecimal(0)
+		// Set default charset and collation instead of binary charset to avoid breaking JSON functions
+		// (see PR #54145 which fixed JSON function argument verification issues).
 		tp.SetCharset(mysql.DefaultCharset)
 		tp.SetCollate(mysql.DefaultCollationName)
 	default: