feat: add 'add column if not exists' for postgres

src/operation-node/column-definition-node.ts

@@ -28,6 +28,7 @@ export interface ColumnDefinitionNode extends OperationNode {
   readonly endModifiers?: ReadonlyArray<OperationNode>
   readonly nullsNotDistinct?: boolean
   readonly identity?: boolean
+  readonly ifNotExists?: boolean
 }
 
 /**

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

@@ -452,6 +452,7 @@ export class OperationNodeTransformer {
       endModifiers: this.transformNodeList(node.endModifiers),
       nullsNotDistinct: node.nullsNotDistinct,
       identity: node.identity,
+      ifNotExists: node.ifNotExists,
     })
   }
 

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

@@ -607,6 +607,10 @@ export class DefaultQueryCompiler
   }
 
   protected override visitColumnDefinition(node: ColumnDefinitionNode): void {
+    if (node.ifNotExists) {
+      this.append('if not exists ')
+    }
+
     this.visitNode(node.column)
 
     this.append(' ')

src/schema/column-definition-builder.ts

@@ -376,6 +376,16 @@ export class ColumnDefinitionBuilder implements OperationNodeSource {
     )
   }
 
+  /**
+   * Adds `if not exists` specifier.
+   * This only works for PostgreSQL.
+   */
+  ifNotExists(): ColumnDefinitionBuilder {
+    return new ColumnDefinitionBuilder(
+      ColumnDefinitionNode.cloneWith(this.#node, { ifNotExists: true }),
+    )
+  }
+
   /**
    * This can be used to add any additional SQL to the end of the column definition.
    *

test/node/src/schema.test.ts

@@ -299,7 +299,7 @@ for (const dialect of DIALECTS) {
           const builder = ctx.db.schema
             .createTable('test')
             .addColumn('a', 'integer', (col) =>
-              col.identity().notNull().primaryKey()
+              col.identity().notNull().primaryKey(),
             )
             .addColumn('b', 'integer', (col) =>
               col
@@ -2329,6 +2329,24 @@ for (const dialect of DIALECTS) {
 
             await builder.execute()
           })
+
+          it('should add a column with "if not exists" modifier', async () => {
+            const builder = ctx.db.schema
+              .alterTable('test')
+              .addColumn('desc', 'varchar(20)', (cb) => cb.ifNotExists())
+
+            testSql(builder, dialect, {
+              postgres: {
+                sql: 'alter table "test" add column if not exists "desc" varchar(20)',
+                parameters: [],
+              },
+              mysql: NOT_SUPPORTED,
+              mssql: NOT_SUPPORTED,
+              sqlite: NOT_SUPPORTED,
+            })
+
+            await builder.execute()
+          })
         }
 
         if (dialect === 'postgres' || dialect === 'mysql') {