add ALTER TABLE drop actions

Sources/PostgreSQL/SQL/PostgreSQLAlterTable.swift

@@ -20,6 +20,62 @@ public struct PostgreSQLAlterTable: SQLAlterTable {
     /// See `SQLAlterTable`.
     public var constraints: [PostgreSQLTableConstraint]
 
+    /// DROP [ COLUMN ] [ IF EXISTS ] column_name [ RESTRICT | CASCADE ]
+    /// DROP CONSTRAINT [ IF EXISTS ]  constraint_name [ RESTRICT | CASCADE ]
+    public struct DropAction: SQLSerializable {
+        public enum Method {
+            case restrict
+            case cascade
+        }
+
+        public enum Kind {
+            case column
+            case constraint
+        }
+
+        public var kind: Kind
+
+        public var ifExists: Bool
+
+        public var column: PostgreSQLIdentifier
+
+        public var method: Method?
+
+        public init(
+            _ kind: Kind,
+            ifExists: Bool = false,
+            _ column: PostgreSQLIdentifier,
+            _ method: Method? = nil
+        ) {
+            self.kind = kind
+            self.ifExists = ifExists
+            self.column = column
+            self.method = method
+        }
+
+        /// See `SQLSerializable`.
+        public func serialize(_ binds: inout [Encodable]) -> String {
+            var sql: [String] = []
+            sql.append("DROP")
+            switch kind {
+            case .column: sql.append("COLUMN")
+            case .constraint: sql.append("CONSTRAINT")
+            }
+            if ifExists {
+                sql.append("IF EXISTS")
+            }
+            sql.append(column.serialize(&binds))
+            if let method = method {
+                switch method {
+                case .cascade: sql.append("CASCADE")
+                case .restrict: sql.append("RESTRICT")
+                }
+            }
+            return sql.joined(separator: " ")
+        }
+    }
+
+    public var dropActions: [DropAction]
 
     /// Creates a new `AlterTable`.
     ///
@@ -29,14 +85,17 @@ public struct PostgreSQLAlterTable: SQLAlterTable {
         self.table = table
         self.columns = []
         self.constraints = []
+        self.dropActions = []
     }
 
     /// See `SQLSerializable`.
     public func serialize(_ binds: inout [Encodable]) -> String {
         var sql: [String] = []
         sql.append("ALTER TABLE")
         sql.append(table.serialize(&binds))
-        let actions = columns.map { "ADD COLUMN " + $0.serialize(&binds) } + constraints.map { "ADD " + $0.serialize(&binds) }
+        let actions = columns.map { "ADD COLUMN " + $0.serialize(&binds) }
+            + constraints.map { "ADD " + $0.serialize(&binds) }
+            + dropActions.map { $0.serialize(&binds) }
         sql.append(actions.joined(separator: ", "))
         return sql.joined(separator: " ")
     }