From b55778a6a34e6b315debe5701b8bf2595be6d611 Mon Sep 17 00:00:00 2001
From: Jerome Haltom <jhaltom@alethic.solutions>
Date: Sat, 12 Sep 2026 10:10:13 -0500
Subject: [PATCH] [CALCITE-7777] JDBC adapter generates || for concatenation,
 which SQL Server rejects

---
 .../org/apache/calcite/sql/dialect/MssqlSqlDialect.java  | 5 +++++
 .../calcite/rel/rel2sql/RelToSqlConverterTest.java       | 9 +++++++++
 2 files changed, 14 insertions(+)

diff --git a/core/src/main/java/org/apache/calcite/sql/dialect/MssqlSqlDialect.java b/core/src/main/java/org/apache/calcite/sql/dialect/MssqlSqlDialect.java
index 964fa03f8f91..7a45559284a0 100644
--- a/core/src/main/java/org/apache/calcite/sql/dialect/MssqlSqlDialect.java
+++ b/core/src/main/java/org/apache/calcite/sql/dialect/MssqlSqlDialect.java
@@ -212,6 +212,11 @@ private static SqlNode createDatetimeCastSpec(String typeAlias, RelDataType type
       final SqlWriter.Frame frame = writer.startFunCall("CEILING");
       call.operand(0).unparse(writer, leftPrec, rightPrec);
       writer.endFunCall(frame);
+    } else if (call.getOperator().equals(SqlStdOperatorTable.CONCAT)) {
+      // MSSQL has no || operator. + concatenates and propagates NULL, as ||
+      // does; the CONCAT function does not, reading a NULL operand as ''.
+      SqlSyntax.BINARY.unparse(writer, SqlStdOperatorTable.PLUS, call, leftPrec,
+          rightPrec);
     } else {
       switch (call.getKind()) {
       case FLOOR:
diff --git a/core/src/test/java/org/apache/calcite/rel/rel2sql/RelToSqlConverterTest.java b/core/src/test/java/org/apache/calcite/rel/rel2sql/RelToSqlConverterTest.java
index 877a1e99ee62..8e43c4400da0 100644
--- a/core/src/test/java/org/apache/calcite/rel/rel2sql/RelToSqlConverterTest.java
+++ b/core/src/test/java/org/apache/calcite/rel/rel2sql/RelToSqlConverterTest.java
@@ -11820,6 +11820,15 @@ private void checkLiteral2(String expression, String expected) {
     sql(query).dialect(MssqlSqlDialect.DEFAULT).ok(mssqlExpected);
   }
 
+  /** Test case for
+   * <a href="https://issues.apache.org/jira/browse/CALCITE-7777">[CALCITE-7777]
+   * JDBC adapter generates || for concatenation, which SQL Server rejects</a>. */
+  @Test void testConcatOperatorEmulationForMSSQL() {
+    final String query = "select \"brand_name\" || \"product_name\" from \"product\"";
+    final String mssqlExpected = "SELECT [brand_name] + [product_name]\n"
+        + "FROM [foodmart].[product]";
+    sql(query).withMssql().ok(mssqlExpected);
+  }
 
   /** Test case for
    * <a href="https://issues.apache.org/jira/browse/CALCITE-6655">[CALCITE-6655]