Dropping unique constraint for column in H2 - sql

Question

I try to drop unique constraint for column in h2, previously created as info varchar(255) unique.
I tried:
sql> alter table public_partner drop constraint (select distinct unique_index_name from in
formation_schema.constraints where table_name='PUBLIC_PARTNER' and column_list='INFO');
But with no success (as follows):
Syntax error in SQL statement "ALTER TABLE PUBLIC_PARTNER DROP CONSTRAINT ([*]SELECT DISTI
NCT UNIQUE_INDEX_NAME FROM INFORMATION_SCHEMA.CONSTRAINTS WHERE TABLE_NAME='PUBLIC_PARTNER
' AND COLUMN_LIST='INFO') "; expected "identifier"; SQL statement:
alter table public_partner drop constraint (select distinct unique_index_name from informa
tion_schema.constraints where table_name='PUBLIC_PARTNER' and column_list='INFO') [42001-1
60]
How this constraint should be correctly removed?
By the way:
sql> (select unique_index_name from information_schema.constraints where table_name='PUBLI
C_PARTNER' and column_list='INFO');
UNIQUE_INDEX_NAME
CONSTRAINT_F574_INDEX_9
(1 row, 0 ms)
seems to return a correct output.

Answer 1

In the SQL language, identifier names can't be expressions. You need to run two statements:
select distinct constraint_name from information_schema.constraints
where table_name='PUBLIC_PARTNER' and column_list='INFO'
and then get the identifier name, and run the statement
ALTER TABLE PUBLIC_PARTNER DROP CONSTRAINT <xxx>

Answer 2

You could use a user defined function to execute a dynamically created statement. First to create the execute alias (only once):
CREATE ALIAS IF NOT EXISTS EXECUTE AS $$ void executeSql(Connection conn, String sql)
throws SQLException { conn.createStatement().executeUpdate(sql); } $$;
Then to call this method:
call execute('ALTER TABLE PUBLIC_PARTNER DROP CONSTRAINT ' ||
(select distinct unique_index_name from in formation_schema.constraints
where table_name='PUBLIC_PARTNER' and column_list='INFO'));
... where execute is the user defined function that runs a statement.

Answer 3

If you are using H2 with Spring Boot in PosgreSQL Mode the query has to include the schema public and the tables are likely in lower case mode. (see application.yml below)
Check the letter case in the information schema table and use the upper and lower case as seen in table information_schema.constraints.
Verbose Query Set
SET #constraint_name = QUOTE_IDENT(
SELECT DISTINCT constraint_name
FROM information_schema.constraints
WHERE table_schema = 'public'
AND table_name = 'public_partner'
AND constraint_type = 'UNIQUE'
AND column_list = 'info');
SET #command = 'ALTER TABLE public.public_partner DROP CONSTRAINT public.' || #constraint_name;
SELECT #command;
EXECUTE IMMEDIATE #command;
Explanation:
SELECT DISTINCT constraint_name [...]
Select the Columns constraint_name with the UNIQUE constrain from the schema info
QUOTE_IDENT([...])
I don't know why this is needed, it will quote the resulting string
SET #constraint_name = [...];
Store in Variable #constraint_name
SET #command = [...];
Compose whole command by concatenation of strings and store in variable #command
SELECT #command;
Show the composed Query on Screen, just for debugging
EXECUTE IMMEDIATE #command;
Execute #command
Typical H2 Configuration in PostgreSQL Mode in the Spring Boot application.yml (extract)
spring:
# [...]
jpa:
database-platform: org.hibernate.dialect.H2Dialect
# [...]
datasource:
url: jdbc:h2:mem:testdb;MODE=PostgreSQL;DATABASE_TO_LOWER=TRUE;DB_CLOSE_DELAY=-1;DB_CLOSE_ON_EXIT=false
username: sa
password: sa
# [...]