feat: add IF EXISTS support for alter table

src/operation-node/alter-table-node.ts

@@ -15,6 +15,7 @@ import type { RenameConstraintNode } from './rename-constraint-node.js'
 
 export type AlterTableNodeTableProps = Pick<
   AlterTableNode,
+  | 'ifExists'
   | 'renameTo'
   | 'setSchema'
   | 'addConstraint'
@@ -34,6 +35,7 @@ export type AlterTableColumnAlterationNode =
 export interface AlterTableNode extends OperationNode {
   readonly kind: 'AlterTableNode'
   readonly table: TableNode
+  readonly ifExists?: boolean
   readonly renameTo?: TableNode
   readonly setSchema?: IdentifierNode
   readonly columnAlterations?: ReadonlyArray<AlterTableColumnAlterationNode>

src/operation-node/operation-node-transformer.ts

@@ -836,6 +836,7 @@ export class OperationNodeTransformer {
     return requireAllProps<AlterTableNode>({
       kind: 'AlterTableNode',
       table: this.transformNode(node.table, queryId),
+      ifExists: node.ifExists,
       renameTo: this.transformNode(node.renameTo, queryId),
       setSchema: this.transformNode(node.setSchema, queryId),
       columnAlterations: this.transformNodeList(

src/query-compiler/default-query-compiler.ts

@@ -1165,6 +1165,11 @@ export class DefaultQueryCompiler
 
   protected override visitAlterTable(node: AlterTableNode): void {
     this.append('alter table ')
+
+    if (node.ifExists) {
+      this.append('if exists ')
+    }
+
     this.visitNode(node.table)
     this.append(' ')
 

src/schema/alter-table-builder.ts

@@ -66,6 +66,20 @@ export class AlterTableBuilder implements ColumnAlteringInterface {
     this.#props = freeze(props)
   }
 
+  /**
+   * Adds the "if exists" modifier.
+   *
+   * If the table doesn't exist, no error is thrown if this method has been called.
+   */
+  ifExists(): AlterTableBuilder {
+    return new AlterTableBuilder({
+      ...this.#props,
+      node: AlterTableNode.cloneWithTableProps(this.#props.node, {
+        ifExists: true,
+      }),
+    })
+  }
+
   renameTo(newTableName: string): AlterTableExecutor {
     return new AlterTableExecutor({
       ...this.#props,

test/node/src/schema.test.ts

@@ -1869,6 +1869,8 @@ for (const dialect of DIALECTS) {
           .createTable('test')
           .addColumn('id', 'bigint', (col) => col.primaryKey())
           .addColumn('first_name', 'varchar(255)')
+          .addColumn('last_name', 'varchar(255)')
+          .addColumn('age', 'integer')
           .execute()
 
         await ctx.db.schema
@@ -2544,6 +2546,31 @@ for (const dialect of DIALECTS) {
           .execute()
       })
 
+      if (dialect === 'postgres' || dialect === 'mysql' || dialect === 'sqlite') {
+        it('should alter table with if exists', async () => {
+          const builder = ctx.db.schema
+            .alterTable('test')
+            .ifExists()
+            .addColumn('date_col', 'date')
+
+          testSql(builder, dialect, {
+            postgres: {
+              sql: 'alter table if exists "test" add column "date_col" date',
+              parameters: [],
+            },
+            mysql: {
+              sql: 'alter table if exists `test` add column `date_col` date',
+              parameters: [],
+            },
+            mssql: NOT_SUPPORTED,
+            sqlite: {
+              sql: 'alter table if exists "test" add column "date_col" date',
+              parameters: [],
+            },
+          })
+        })
+      }
+
       describe('add column', () => {
         it('should add a column', async () => {
           const builder = ctx.db.schema