I am using mysql db. I know postgresql and SQL server supports partial Indexing. In my case I want to do something like this:
CREATE UNIQUE INDEX myIndex ON myTable (myColumn) where myColumn <> 'myText'
I want to create a unique constraint but it should allow duplicates if it is a particular text.
I couldn't find a direct way to do this in mysql. But, is there a workaround to achieve it?
Filtered indexes could be emulated with function indexes and CASE
expression(MySQL 8.0.13 and newer):
CREATE TABLE t(id INT PRIMARY KEY, myColumn VARCHAR(100));
-- NULL are not taken into account with `UNIQUE` indexes
CREATE UNIQUE INDEX myIndex ON t((CASE WHEN myColumn <> 'myText' THEN myColumn END));
-- inserting excluded value twice
INSERT INTO t(id, myColumn) VALUES(1, 'myText'), (2, 'myText');
-- trying to insert different value than excluded twice
INSERT INTO t(id, myColumn) VALUES(3, 'aaaaa');
INSERT INTO t(id, myColumn) VALUES(4, 'aaaaa');
-- Duplicate entry 'aaaaa' for key 'myIndex'
SELECT * FROM t;
db<>fiddle demo
Output:
+-----+----------+
| id | myColumn |
+-----+----------+
| 1 | myText |
| 2 | myText |
| 3 | aaaaa |
+-----+----------+
I suppose there is only one way to achieve it. You can add another column to your table, create index on it and create trigger or do insert/update inside your stored procedures to fill this column using following condition:
if value = 'myText' then put null
otherwise put value
Hope it helps
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