Error: there is no unique constraint matching given keys for referenced table "bar"

Necessity of unique constraints

In the world of PostgreSQL, unique constraints are the lifeblood that maintain the integrity of relationships between tables. Like a good love story, each record must be unique and identifiable. The drama unfolds if foo references bar and lands on duplicate values—cheesy, but conscious.

To avoid this, unique constraints help your database to maintain unambiguous relationships and keep data anomalies at bay.

Fixing the table schema

Now, let's ring some wedding bells between these tables. You have two main options:

• Declare love at first sight with a column-level unique constraint: this treats a single column to a unique status:

  CREATE TABLE bar (
      bar_id SERIAL PRIMARY KEY,
      referenced_column VARCHAR(255) UNIQUE -- Friends don't let friends have duplicates!
  );
  

• Or, play the matchmaker with a table-level unique constraint: this ensures a unique combination of columns:

  CREATE TABLE bar (
      bar_id SERIAL PRIMARY KEY,
      col1 VARCHAR(255),
      col2 VARCHAR(255),
      UNIQUE (col1, col2) -- Like salt and pepper, they're unique together
  );
  

Troubleshooting your relationship with constraints

SQL syntax: your love language:

PostgreSQL is particular about its love language (aka SQL syntax). Minor mistakes might result in big mishaps, like being dumped... out of the database.

Resolving pre-existing drama:

You can't invite a unique constraint into a space that still has ongoing drama (a.k.a duplicate values). Clean up the past before moving forward.

Not forgetting your exes... I mean, keys:

Remember, in your schema design, a foreign key relationship should always be around unique identifiers.

Perfecting the romance: Advanced tips

Brewing the perfect coffee, or, err... index:

A unique constraint is like a barista: it brews up a unique index immediately. The index is your espresso shot: it enforces uniqueness and packs a powerful punch to query performance.

Anticipating heartbreaks:

Adding a unique constraint is a locking operation—you're making a commitment. Handle high-traffic tables with diligence and care; it might get emotional.

Planning for future twists:

The behavior of cascades is that little je ne sais quoi that can greatly affect your data. Remember, in love and in PostgreSQL tables—always plan for the unexpected.