Add support for parsing MsSql alias with equals (#1467)

Author: yoavcloud

src/dialect/mod.rs

@@ -561,6 +561,17 @@ pub trait Dialect: Debug + Any {
     fn supports_asc_desc_in_column_definition(&self) -> bool {
         false
     }
+
+    /// Returns true if this dialect supports treating the equals operator `=` within a `SelectItem`
+    /// as an alias assignment operator, rather than a boolean expression.
+    /// For example: the following statements are equivalent for such a dialect:
+    /// ```sql
+    ///  SELECT col_alias = col FROM tbl;
+    ///  SELECT col_alias AS col FROM tbl;
+    /// ```
+    fn supports_eq_alias_assigment(&self) -> bool {
+        false
+    }
 }
 
 /// This represents the operators for which precedence must be defined

src/dialect/mssql.rs

@@ -49,4 +49,8 @@ impl Dialect for MsSqlDialect {
     fn supports_connect_by(&self) -> bool {
         true
     }
+
+    fn supports_eq_alias_assigment(&self) -> bool {
+        true
+    }
 }

src/parser/mod.rs

@@ -11181,6 +11181,24 @@ impl<'a> Parser<'a> {
                     self.peek_token().location
                 )
             }
+            Expr::BinaryOp {
+                left,
+                op: BinaryOperator::Eq,
+                right,
+            } if self.dialect.supports_eq_alias_assigment()
+                && matches!(left.as_ref(), Expr::Identifier(_)) =>
+            {
+                let Expr::Identifier(alias) = *left else {
+                    return parser_err!(
+                        "BUG: expected identifier expression as alias",
+                        self.peek_token().location
+                    );
+                };
+                Ok(SelectItem::ExprWithAlias {
+                    expr: *right,
+                    alias,
+                })
+            }
             expr => self
                 .parse_optional_alias(keywords::RESERVED_FOR_COLUMN_ALIAS)
                 .map(|alias| match alias {

tests/sqlparser_common.rs

@@ -11432,3 +11432,20 @@ fn test_any_some_all_comparison() {
     verified_stmt("SELECT c1 FROM tbl WHERE c1 <> SOME(SELECT c2 FROM tbl)");
     verified_stmt("SELECT 1 = ANY(WITH x AS (SELECT 1) SELECT * FROM x)");
 }
+
+#[test]
+fn test_alias_equal_expr() {
+    let dialects = all_dialects_where(|d| d.supports_eq_alias_assigment());
+    let sql = r#"SELECT some_alias = some_column FROM some_table"#;
+    let expected = r#"SELECT some_column AS some_alias FROM some_table"#;
+    let _ = dialects.one_statement_parses_to(sql, expected);
+
+    let sql = r#"SELECT some_alias = (a*b) FROM some_table"#;
+    let expected = r#"SELECT (a * b) AS some_alias FROM some_table"#;
+    let _ = dialects.one_statement_parses_to(sql, expected);
+
+    let dialects = all_dialects_where(|d| !d.supports_eq_alias_assigment());
+    let sql = r#"SELECT x = (a * b) FROM some_table"#;
+    let expected = r#"SELECT x = (a * b) FROM some_table"#;
+    let _ = dialects.one_statement_parses_to(sql, expected);
+}