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## Environment
**Liquibase Version**: 4.2.2

**Liquibase Integration & Version**: Maven

**Liquibase Extension(s) & Version**: NA

**Database Vendor & Version**: com.ibm.db2:db2jcc_license_cisuz & 9.7.5

**Operating System Type & Version**: Windows 7 Enterprise

## Description
I tried to implement Liquibase for my IBM DB2 zos database and when i run liquibase:generateChangeLog command, I’m getting the following error.

Error setting up or running Liquibase: liquibase.command.CommandExcecutionExceptoin: liquibase.exception.DatabaseException: Error executing SQL select CONSTRAINT_NAME, COLUMN_LIST as COLUMN_NAME, constraint_schema as CONSTRAINT_CONTAINER from information_schema.constraints where constraint_type=‘UNIQUE’ and …; DB2 SQL Error: SQLCODE=-204, SQLSTATE=42704, SQLERRMC=INFORMATION_SCHEMA.CONSTRAINTS, DRIVER=4.25.13

SQLCODE=-204, object not found, Basically the table/schema is not found.

When I ran the same command in debug mode, I could see that in the following class, liquibase/UniqueConstraintSnapshotGenerator.java at master · liquibase/liquibase · GitHub there is no logic to handle DB2z and hence it is considering no specific handler for RDBMS was provided and assuming INFORMATION_SCHEMA.

We do have logic for DB2Database, but not DB2ZDatabase .

## Steps To Reproduce
1. Create a maven project
1. Add the following dependencies in pom.xml
  * com.jbm.db2:db2jcc4 & 4.25.13
  * com.ibm.db2:db2jcc_license_cisuz & 9.7.5
1. Add the following plugin to pom.xml
  * org.liquibase:liquibase-maven-plugin & 4.2.2
  * in the <configuration> tag, add <propertyFile> and specify the property file liquibase-dev.properties to run
1. In the property file liquibase-dev.properties, I specified the follwoing
  * url(jdbc:db2://<hostname>:<port>/currentSchema=<schemaName>;) , username, password, ouputChangeLogFile, driver(com.ibm.db2.jcc.DB2Driver).
1. Run mvn clean and run mvn liquibase:generateChangeLog

## Actual Behavior
When I run mvn liquibase:genereateChangeLog, I'm getting the below error,

Error setting up or running Liquibase: liquibase.command.CommandExcecutionExceptoin: liquibase.exception.DatabaseException: Error executing SQL select CONSTRAINT_NAME, COLUMN_LIST as COLUMN_NAME, constraint_schema as CONSTRAINT_CONTAINER from information_schema.constraints where constraint_type=‘UNIQUE’ and …; DB2 SQL Error: SQLCODE=-204, SQLSTATE=42704, SQLERRMC=INFORMATION_SCHEMA.CONSTRAINTS, DRIVER=4.25.13

## Expected/Desired Behavior
mvn liquibase:generateChangeLog should generate the change log for IBM DB2z based on the current specified database schema.

## Screenshots (if appropriate)
If applicable, add screenshots to help explain your problem.

## Additional Context
Add any other context about the problem here.


Map rowData = rows.get(0);
                    String descriptor = rowData.get("DESCRIPTOR").toString();
                    descriptor = descriptor.replaceFirst(".*\\(", "").replaceFirst("\\).*", "");
                    for (String columnNumber : StringUtil.splitAndTrim(descriptor, ",")) {
                        String columnName = Scope.getCurrentScope().getSingleton(ExecutorService.class).getExecutor("jdbc", database).queryForObject(new RawSqlStatement(
                                "select c.columnname from sys.syscolumns c "
                                        + "join sys.systables t on t.tableid=c.referenceid "
                                        + "where t.tablename='" + rowData.get("TABLENAME") + "' and c.columnnumber=" + columnNumber), String.class);
                        Map<String, String> row = new HashMap<>();
                        row.put("COLUMN_NAME", columnName);
                        returnList.add(row);
                    }
                    return returnList;
                }
            } else if (database instanceof FirebirdDatabase) {
                
                
                sql = "SELECT TRIM(RDB$INDEX_SEGMENTS.RDB$FIELD_NAME) AS column_name " +
                        "FROM RDB$INDEX_SEGMENTS " +
                        "LEFT JOIN RDB$INDICES ON RDB$INDICES.RDB$INDEX_NAME = RDB$INDEX_SEGMENTS.RDB$INDEX_NAME " +
                        "WHERE UPPER(TRIM(RDB$INDICES.RDB$INDEX_NAME))='" + database.correctObjectName(name, UniqueConstraint.class) + "' " +
                        "ORDER BY RDB$INDEX_SEGMENTS.RDB$FIELD_POSITION";
            } else if (database instanceof SybaseASADatabase) {
                
                sql = "select sysconstraint.constraint_name, syscolumn.column_name " +
                        "from sysconstraint, syscolumn, systable " +
                        "where sysconstraint.ref_object_id = syscolumn.object_id " +
                        "and sysconstraint.table_object_id = systable.object_id " +
                        "and sysconstraint.constraint_name = '" + database.correctObjectName(name, UniqueConstraint.class) + "' " +
                        "and systable.table_name = '" + database.correctObjectName(example.getRelation().getName(), Table.class) + "'";