Implement ALTER TABLE DROP COLUMN (#148)

This implements `DROP [ COLUMN ] [ IF EXISTS ] column_name [ CASCADE ]`
sub-command of `ALTER TABLE`, which is what PostgreSQL supports https://www.postgresql.org/docs/12/sql-altertable.html
(except for the RESTRICT option)

Co-authored-by: Nickolay Ponomarev <asqueella@gmail.com>

CHANGELOG.md

@@ -31,6 +31,7 @@ Check https://github.com/andygrove/sqlparser-rs/commits/master for undocumented
 - Support bitwise AND (`&`), OR (`|`), XOR (`^`) (#181) - thanks @Dandandan!
 - Add serde support to AST structs and enums (#196) - thanks @panarch!
 - Support `ALTER TABLE ADD COLUMN`, `RENAME COLUMN`, and `RENAME TO` (#203) - thanks @mashuai!
+- Support `ALTER TABLE DROP COLUMN` (#148) - thanks @ivanceras!
 
 ### Fixed
 - Report an error for unterminated string literals (#165)

src/ast/ddl.rs

@@ -27,6 +27,12 @@ pub enum AlterTableOperation {
     AddColumn { column_def: ColumnDef },
     /// TODO: implement `DROP CONSTRAINT <name>`
     DropConstraint { name: Ident },
+    /// `DROP [ COLUMN ] [ IF EXISTS ] <column_name> [ CASCADE ]`
+    DropColumn {
+        column_name: Ident,
+        if_exists: bool,
+        cascade: bool,
+    },
     /// `RENAME [ COLUMN ] <old_column_name> TO <new_column_name>`
     RenameColumn {
         old_column_name: Ident,
@@ -44,6 +50,17 @@ impl fmt::Display for AlterTableOperation {
                 write!(f, "ADD COLUMN {}", column_def.to_string())
             }
             AlterTableOperation::DropConstraint { name } => write!(f, "DROP CONSTRAINT {}", name),
+            AlterTableOperation::DropColumn {
+                column_name,
+                if_exists,
+                cascade,
+            } => write!(
+                f,
+                "DROP COLUMN {}{}{}",
+                if *if_exists { "IF EXISTS " } else { "" },
+                column_name,
+                if *cascade { " CASCADE" } else { "" }
+            ),
             AlterTableOperation::RenameColumn {
                 old_column_name,
                 new_column_name,

src/parser.rs

@@ -1340,8 +1340,18 @@ impl Parser {
                     new_column_name,
                 }
             }
+        } else if self.parse_keyword(Keyword::DROP) {
+            let _ = self.parse_keyword(Keyword::COLUMN);
+            let if_exists = self.parse_keywords(&[Keyword::IF, Keyword::EXISTS]);
+            let column_name = self.parse_identifier()?;
+            let cascade = self.parse_keyword(Keyword::CASCADE);
+            AlterTableOperation::DropColumn {
+                column_name,
+                if_exists,
+                cascade,
+            }
         } else {
-            return self.expected("ADD or RENAME after ALTER TABLE", self.peek_token());
+            return self.expected("ADD, RENAME, or DROP after ALTER TABLE", self.peek_token());
         };
         Ok(Statement::AlterTable {
             name: table_name,

tests/sqlparser_common.rs

@@ -1388,6 +1388,39 @@ fn parse_alter_table_constraints() {
     }
 }
 
+#[test]
+fn parse_alter_table_drop_column() {
+    check_one("DROP COLUMN IF EXISTS is_active CASCADE");
+    one_statement_parses_to(
+        "ALTER TABLE tab DROP IF EXISTS is_active CASCADE",
+        "ALTER TABLE tab DROP COLUMN IF EXISTS is_active CASCADE",
+    );
+    one_statement_parses_to(
+        "ALTER TABLE tab DROP is_active CASCADE",
+        "ALTER TABLE tab DROP COLUMN is_active CASCADE",
+    );
+
+    fn check_one(constraint_text: &str) {
+        match verified_stmt(&format!("ALTER TABLE tab {}", constraint_text)) {
+            Statement::AlterTable {
+                name,
+                operation:
+                    AlterTableOperation::DropColumn {
+                        column_name,
+                        if_exists,
+                        cascade,
+                    },
+            } => {
+                assert_eq!("tab", name.to_string());
+                assert_eq!("is_active", column_name.to_string());
+                assert_eq!(true, if_exists);
+                assert_eq!(true, cascade);
+            }
+            _ => unreachable!(),
+        }
+    }
+}
+
 #[test]
 fn parse_bad_constraint() {
     let res = parse_sql_statements("ALTER TABLE tab ADD");