Add support for renaming columns within a table

SQLite introduced enhancements to the `ALTER TABLE` command in 3.25.0.
iOS 13.0+, macOS 10.15+, tvOS 13.0+, and watchOS 6.0+ have a SQLite
library bundled higher than 3.25.0.

For more information see https://www.sqlite.org/lang_altertable.html

GRDB/QueryInterface/Schema/TableDefinition.swift

@@ -657,6 +657,7 @@ public final class TableDefinition {
 public final class TableAlteration {
     private let name: String
     private var addedColumns: [ColumnDefinition] = []
+    private var renamedColumns: [(oldColumn: ColumnDefinition, newColumn: ColumnDefinition)] = []
 
     init(name: String) {
         self.name = name
@@ -680,9 +681,42 @@ public final class TableAlteration {
         addedColumns.append(column)
         return column
     }
+
+    #if !os(OSX)
+    /// Renames a column in a table.
+    ///
+    ///     try db.alter(table: "player") { t in
+    ///         t.rename(column: "url", to: "home_url")
+    ///     }
+    ///
+    /// See https://www.sqlite.org/lang_altertable.html
+    ///
+    /// - parameter name: the column name to rename.
+    /// - parameter newName: the new name of the column.
+    @available(iOS 13.0, tvOS 13.0, watchOS 6.0, *)
+    public func rename(column name: String, to newName: String) {
+        let old = ColumnDefinition(name: name, type: nil)
+        let new = ColumnDefinition(name: newName, type: nil)
+        renamedColumns.append((oldColumn: old, newColumn: new))
+    }
+    #endif
 
     fileprivate func sql(_ db: Database) throws -> String {
         var statements: [String] = []
+
+        #if !os(OSX)
+        for (oldColumn, newColumn) in renamedColumns {
+            var chunks: [String] = []
+            chunks.append("ALTER TABLE")
+            chunks.append(name.quotedDatabaseIdentifier)
+            chunks.append("RENAME COLUMN")
+            chunks.append(oldColumn.name.quotedDatabaseIdentifier)
+            chunks.append("TO")
+            chunks.append(newColumn.name.quotedDatabaseIdentifier)
+            let statement = chunks.joined(separator: " ")
+            statements.append(statement)
+        }
+        #endif
 
         for column in addedColumns {
             var chunks: [String] = []

Tests/GRDBTests/TableDefinitionTests.swift

@@ -495,6 +495,27 @@ class TableDefinitionTests: GRDBTestCase {
             assertEqualSQL(sqlQueries[sqlQueries.count - 1], "ALTER TABLE \"test\" ADD COLUMN \"e\"")
         }
     }
+
+    #if !os(OSX)
+    @available(iOS 13.0, tvOS 13.0, watchOS 6.0, *)
+    func testAlterTableRenameColumn() throws {
+        let dbQueue = try makeDatabaseQueue()
+        try dbQueue.inDatabase { db in
+            try db.create(table: "test") { t in
+                t.column("a", .text)
+            }
+
+            sqlQueries.removeAll()
+            try db.alter(table: "test") { t in
+                t.rename(column: "a", to: "b")
+                t.add(column: "e")
+            }
+
+            assertEqualSQL(sqlQueries[sqlQueries.count - 2], "ALTER TABLE \"test\" RENAME COLUMN \"a\" TO \"b\"")
+            assertEqualSQL(sqlQueries[sqlQueries.count - 1], "ALTER TABLE \"test\" ADD COLUMN \"e\"")
+        }
+    }
+    #endif
 
     func testDropTable() throws {
         let dbQueue = try makeDatabaseQueue()