Cannot alter table (add unique constraint) in a SQL PL store procedure for db2

I am trying to create a store procedure in SQL PL for db2. A new field needs to be added as part of the unique key, so I need to drop the previous uk first, and then add the new unique key. That worked fine for MS SQLServer and Oracle, but I cannot make it for db2.

CREATE PROCEDURE update_unique_key ()
LANGUAGE SQL
BEGIN 
    DECLARE uk_constraint_name VARCHAR(50);
    DECLARE sql_stmt VARCHAR(100);  
    SELECT constname INTO uk_constraint_name FROM SYSCAT.TABCONST WHERE tabname = 'TABLE_NAME' AND type = 'U';
    SET sql_stmt = 'alter table TABLE_NAME drop constraint ' || char(uk_constraint_name);
    开发者_Go百科SET sql_stmt_2 = 'alter table TABLE_NAME add unique (F1, F2)';  

    prepare s1 from sql_stmt;
    prepare s2 from sql_stmt_2;

    EXECUTE IMMEDIATE s1;   
    EXECUTE IMMEDIATE s2;       
END

Getting this error when trying to deploy the routine with IBM Data Studio as client (the database is db2 9.5)

    DEV.UPDATE_UNIQUE_KEY - Deploy started.
Create stored procedure returns SQLCODE: -206, SQLSTATE: 42703.
DEV.UPDATE_UNIQUE_KEY: 8: "SQL_STMT_2" is not valid in the context where it is used.. SQLCODE=-206, SQLSTATE=42703, DRIVER=4.12.55
"SQL_STMT_2" is not valid in the context where it is used.. SQLCODE=-206, SQLSTATE=42703, DRIVER=4.12.55

The thing I don't understand is:

- If in the routine I just drop the constraint it works.

- I can add the constraint just doing an alter table from a sql client (Squirrel)

- If I have just a routine trying to add the constraint I get the same error.

I appreciate any help. Thanks

I just forgot to declare the variable... DECLARE sql_stmt_2 VARCHAR(100);