Mysql – Multi-field unique constraint where we include one or another field based on nullity

You can do that in pure SQL. Create a partial unique index in addition to the one you have:

CREATE UNIQUE INDEX ab_c_null_idx ON my_table (id_A, id_B) WHERE id_C IS NULL;

This way you can enter for (id_A, id_B, id_C) in your table:

(1, 2, 1)
(1, 2, 2)
(1, 2, NULL)

But none of these a second time.

Or use two partial UNIQUE indexes and no complete index (or constraint). The best solution depends on the details of your requirements. Compare:

• Create unique constraint with null columns

While this is elegant and efficient for a single nullable column in the UNIQUE index, it gets out of hand quickly for more than one. Discussing this - and how to use UPSERT with partial indexes:

• PostgreSQL UPSERT issue with NULL values

Asides

No use for mixed case identifiers without double quotes in PostgreSQL.

You might consider a serial column as primary key or an IDENTITY column in Postgres 10 or later. Related:

• Auto increment table column

So:

CREATE TABLE my_table (
   my_table_id bigint GENERATED BY DEFAULT AS IDENTITY PRIMARY KEY  -- for pg 10+
-- my_table_id bigserial PRIMARY KEY  -- for pg 9.6 or older
 , id_a int8 NOT NULL
 , id_b int8 NOT NULL
 , id_c int8
 , CONSTRAINT u_constraint UNIQUE (id_a, id_b, id_c)
);

If you don't expect more than 2 billion rows (> 2147483647) over the lifetime of your table (including waste and deleted rows), consider integer (4 bytes) instead of bigint (8 bytes).

In other words, you want values in the column subset to be unique among rows where the column type is 'true'.
A partial unique index will do that:

CREATE UNIQUE INDEX tbl_some_name_idx ON tbl (subset) WHERE type = 'true';

Data type does matter. If the column type is boolean (likely should be), you can simplify:

CREATE UNIQUE INDEX tbl_some_name_idx ON tbl (subset) WHERE type;

This way you can even make combinations with NULL unique, which is not possible otherwise. See:

• PostgreSQL multi-column unique constraint and NULL values