Hvorfor fortæller MySQL Workbench mig, at jeg har brug for et semikolon?

Jeg tror, ​​problemet er:du bruger ikke DELIMITER .

Så siger det bare sådan:

DELIMITER //
create procedure AddColumnUnlessExists(
    IN dbName tinytext,
    IN tableName tinytext,
    IN fieldName tinytext,
    IN fieldDef text)
begin
    IF NOT EXISTS (
        SELECT * FROM information_schema.COLUMNS
        WHERE column_name=fieldName
        and table_name=tableName
        and table_schema=dbName
        )
    THEN
        set @ddl = CONCAT('ALTER TABLE ', dbName, '.', tableName, ' ADD COLUMN ', fieldName, ' ', fieldDef);
        prepare stmt from @ddl;
        execute stmt;
    END IF;
end //
DELIMITER ;

REDIGER https://dev.mysql.com/doc /refman/5.7/da/stored-programs-definining.html