Alter TABLE to add a composite primary key

Handling prior keys: The not so easy part

Before associating a new composite primary key, be certain that there are no conflicting keys. If there's a previously defined primary key, drop it using the following method:

/* Does it feel like breaking up...with your primary key? */
ALTER TABLE table
DROP PRIMARY KEY;
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Having dropped the existing primary key, you're clear to add your shiny new composite key. And remember, composite columns offer unique identifiability.

Whenever your keys open other doors

Does your primary key have a life outside of your table? If it's referenced as a foreign key in another table, drop the foreign key constraint before you alter your primary key.

ALTER TABLE other_table
DROP FOREIGN KEY fk_name; /* Adieu, foreign key. We barely knew ye */
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After modifying the primary key, don't forget to redefine those foreign keys to be in sync with the new structure.

A throne not for two: When performance is at stake

The order of columns in your composite key bestows a performance boon or bane on thee. The database seeks out columns for indexing sequentially, hence the most sought-after column should don the crown:

ALTER TABLE table ADD CONSTRAINT key_name PRIMARY KEY (columnA, columnB); /* All hail, columnA */
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Considering creating a surrogate key for simpler key governance? AUTO_INCREMENT field can serve as your own Littlefinger.

All keys on deck: Ensuring unique combination

Still doubting whether columnA and columnB combination ensures uniqueness? Cross-examining can spare you from unwittingly generating duplicate rows, camouflaged amidst unique ones.

The Other Folk: Unique constraints

In some instances, a UNIQUE CONSTRAINT could serve you better than a primary key. It lets you establish a composite key that doesn't hog the primary identifier spotlight:

ALTER TABLE table
ADD CONSTRAINT constraint_name UNIQUE (columnA, columnB); /* Feel special with the UNIQUE constraint, no pressure! */
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Naming your keys: a Game of Tables

While defining a new constraint, it's important to christen them properly. Consistent naming paves the way for better maintainability – all while reducing variations:

ALTER TABLE table
ADD CONSTRAINT pk_table_columnA_columnB PRIMARY KEY (columnA, columnB); /* Move over Jon Snow, this is the true heir */
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In here, pk_ signifies primary key, succeeded by the table name and included column names.

Potential hurdles on the path

Large composite keys: Taming the dragons

Dealing with tables rocking multiple columns? Composite keys can ratchet up the complexity. Regular review and normalization can help keep your tables fit and lean.

Join operations: Dance of the Tables

More columns in a primary key equals more complex JOIN expressions. This juggling act can impact readability, and turn the spotlight on performance. It's a delicate waltz between the normalization and practicality of your queries.

Data integrity: Keeping the peace in Westeros

Altering primary keys can trigger a domino effect – leading to cascading updates or deletions that can turn your tables, uh, upside down. Always backup your data before starting the war.