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+---
+title: Add Constraints Safely in Migrations
+impact: HIGH
+impactDescription: Prevents migration failures and enables idempotent schema changes
+tags: constraints, migrations, schema, alter-table
+---
+
+## Add Constraints Safely in Migrations
+
+PostgreSQL does not support `ADD CONSTRAINT IF NOT EXISTS`. Migrations using this syntax will fail.
+
+**Incorrect (causes syntax error):**
+
+```sql
+-- ERROR: syntax error at or near "not" (SQLSTATE 42601)
+alter table public.profiles
+add constraint if not exists profiles_birthchart_id_unique unique (birthchart_id);
+```
+
+**Correct (idempotent constraint creation):**
+
+```sql
+-- Use DO block to check before adding
+do $$
+begin
+  if not exists (
+    select 1 from pg_constraint
+    where conname = 'profiles_birthchart_id_unique'
+    and conrelid = 'public.profiles'::regclass
+  ) then
+    alter table public.profiles
+    add constraint profiles_birthchart_id_unique unique (birthchart_id);
+  end if;
+end $$;
+```
+
+For all constraint types:
+
+```sql
+-- Check constraints
+do $$
+begin
+  if not exists (
+    select 1 from pg_constraint
+    where conname = 'check_age_positive'
+  ) then
+    alter table users add constraint check_age_positive check (age > 0);
+  end if;
+end $$;
+
+-- Foreign keys
+do $$
+begin
+  if not exists (
+    select 1 from pg_constraint
+    where conname = 'profiles_birthchart_id_fkey'
+  ) then
+    alter table profiles
+    add constraint profiles_birthchart_id_fkey
+    foreign key (birthchart_id) references birthcharts(id);
+  end if;
+end $$;
+```
+
+Check if constraint exists:
+
+```sql
+-- Query to check constraint existence
+select conname, contype, pg_get_constraintdef(oid)
+from pg_constraint
+where conrelid = 'public.profiles'::regclass;
+
+-- contype values:
+-- 'p' = PRIMARY KEY
+-- 'f' = FOREIGN KEY
+-- 'u' = UNIQUE
+-- 'c' = CHECK
+```
+
+Reference: [Constraints](https://www.postgresql.org/docs/current/ddl-constraints.html)