chore: update migration content for sample db

sample/database/migrations/210206002058_create_tag/down.sql

@@ -1,2 +1,3 @@
 -- This file should undo anything in `up.sql`
 
+DROP TABLE tags;

sample/database/migrations/210206002058_create_tag/up.sql

@@ -0,0 +1,7 @@
+-- Your SQL goes here
+
+CREATE TABLE tags (
+    id          serial          PRIMARY KEY,
+    name        text            NOT NULL UNIQUE CHECK(length(name) > 0),
+    created_at  timestamp       NOT NULL DEFAULT current_timestamp
+);

sample/database/migrations/210206002058_hello_world/up.sql

@@ -1,2 +0,0 @@
--- Your SQL goes here
-

sample/database/migrations/210206002359_second_migration/down.sql

@@ -1,2 +0,0 @@
--- This file should undo anything in `up.sql`
-

sample/database/migrations/210206002359_second_migration/up.sql

@@ -1,2 +0,0 @@
--- Your SQL goes here
-