Dropping unique constraint for column in H2

问题

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.

回答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>

回答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.

来源：https://stackoverflow.com/questions/10008476/dropping-unique-constraint-for-column-in-h2

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