Liquibase: <changeSet> with multiple SQL statements in <sql> using parameters - mysql

Question

I need to run a prepared statement in Liquibase:
<changeSet id="53" author="foo">
<sql dbms="!h2, mysql" splitStatements="true">
SET #table_name = 'AUDIT_EVENT';
SET #column_name = 'ACTOR_ID';
SET #constraint_name = (
SELECT rc.CONSTRAINT_NAME
FROM INFORMATION_SCHEMA.REFERENTIAL_CONSTRAINTS AS rc
INNER JOIN INFORMATION_SCHEMA.KEY_COLUMN_USAGE AS kcu
ON kcu.CONSTRAINT_CATALOG = rc.CONSTRAINT_CATALOG
AND kcu.CONSTRAINT_NAME = rc.CONSTRAINT_NAME
WHERE kcu.TABLE_NAME = #table_name
AND kcu.COLUMN_NAME = #column_name
);
SET #s = concat('ALTER TABLE ', #table_name, ' DROP FOREIGN KEY ', #constraint_name);
PREPARE stmt FROM #s;
EXECUTE stmt;
DEALLOCATE PREPARE stmt
</sql>
</changeSet>
If I set splitStatements to true, then I think parameters don't persist across statements so understandably I get an error saying that #s is NULL:
Reason: liquibase.exception.DatabaseException: You have an error in your SQL syntax;
check the manual that corresponds to your MySQL server version for the right syntax
to use near 'NULL' at line 1 [Failed SQL: PREPARE stmt FROM #s]
If I set splitStatements to false, and also delete all but the first two SETs to make sure it's not the nested SELECT causing the issue, I get this:
Reason: liquibase.exception.DatabaseException: You have an error in your SQL syntax;
check the manual that corresponds to your MySQL server version for the right syntax
to use near 'SET #column_name = 'ACTOR_ID'' at line 2 [Failed SQL: SET #table_name = 'AUDIT_EVENT';
SET #column_name = 'ACTOR_ID';]
I assumed then the issue with with the endDelimiter defaulting to ;. So I tried FOO:
<sql dbms="!h2, mysql" splitStatements="false" endDelimiter="FOO">
SET #table_name = 'AUDIT_EVENT';
SET #column_name = 'ACTOR_ID';
FOO
</sql>
To which I get...
Reason: liquibase.exception.DatabaseException: You have an error in your SQL syntax;
check the manual that corresponds to your MySQL server version for the right syntax
to use near 'SET #column_name = 'ACTOR_ID';
FOO' at line 2 [Failed SQL: SET #table_name = 'AUDIT_EVENT';
SET #column_name = 'ACTOR_ID';
FOO]
I feel like I don't understand what's going on. I checked SQL does work in MySQL Workbench. Any insights?

Answer 1

Update
So, I thought splitStatements=false got me past the NULL #s error, and therefore hypothesized that parameters (or "user/session variables" as I now know to call them) must not persist when split. But that was all wrong. Those variables do persist across statements: for the duration of the session.
splitStatements=false simply masked the original error with a new one, which did indeed have to do with MySQL not allowing multiple queries at once. When I unblocked that using allowMultiQueries=true, I got a new error pointing to Line 1, which eventually got me to fix the original error, which was that my table name was wrong (uppercase instead of lowercase).
What I never realized was that, the table name being wrong also contributed to #s ending up NULL.
Old "Solution"
I didn't realize that MySQL doesn't allow multiple queries by default. I had to add that in the JDBC URL as a query parameter:
jdbc:mysql://%s:%d/%s?user=root&allowMultiQueries=true
As I understand, this is not a portable path. But since I'm using Liquibase to specify exactly what database to run these statements on, I think it's okay.