I'm learning from an online course MySQL using the WampServer and editing code in MySQL Workbench.
Trying to create a table with the following code, the column nome appears to be the primary key from the table, but i'm only using the unique constraint. When i don't use the unique constraint, the code runs normally and don't give me a primary key.
create table if not exists cursos (
nome varchar(30) not null unique,
descricao text,
carga int unsigned,
totalaulas int unsigned,
ano year default '2016'
) default charset utf8mb4;
Second to this question, when i was trying to drop the constraint primary key i was getting the error Error Code: 1091. Can't DROP 'PRIMARY';
alter table cursos
drop primary key;
So, in resume, i'm trying to use the unique constraint without setting a column has a primary key, and then i'm trying to drop the primary key constraint.
Edit:
When i call describe table i get this, the column nome is defined has primary key whithout i using the constraint.
+------------------+-------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+------------------+-------------+------+-----+---------+----------------+
| idcurso | int | YES | | NULL | |
| nome | varchar(30) | NO | PRI | NULL | |
+------------------+-------------+------+-----+---------+----------------+
What i'm trying to do is drop the primary key from nome and put on idcurso, but when i use the code
alter table cursos
add primary key idcurso;
I get the error Error Code: 1064. 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 '' at line
I tried this on mysql command-line client connected to a MySQL Server 5.5.2. Here is what happens:
mysql> create table if not exists cursos (
-> nome varchar(30) not null unique,
-> descricao text,
-> carga int unsigned,
-> totalaulas int unsigned,
-> ano year default '2016'
-> ) default charset utf8mb4;
Query OK, 0 rows affected (0.08 sec)
mysql>
mysql> desc cursos;
+------------+------------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+------------+------------------+------+-----+---------+-------+
| nome | varchar(30) | NO | PRI | NULL | |
| descricao | text | YES | | NULL | |
| carga | int(10) unsigned | YES | | NULL | |
| totalaulas | int(10) unsigned | YES | | NULL | |
| ano | year(4) | YES | | 2016 | |
+------------+------------------+------+-----+---------+-------+
5 rows in set (0.05 sec)
Note that the field nome has a Key as PRI. But, it is only a description and not a fact. If you try to DROP the primary key you will see an error like below:
mysql> alter table cursos drop primary key;
ERROR 1091 (42000): Can't DROP 'PRIMARY'; check that column/key exists
Add a new column and make it a primary key. Note the DESCRIPTION after that.
mysql> alter table cursos add idcurso int primary key;
Query OK, 0 rows affected (0.09 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> desc cursos;
+------------+------------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+------------+------------------+------+-----+---------+-------+
| nome | varchar(30) | NO | UNI | NULL | |
| descricao | text | YES | | NULL | |
| carga | int(10) unsigned | YES | | NULL | |
| totalaulas | int(10) unsigned | YES | | NULL | |
| ano | year(4) | YES | | 2016 | |
| idcurso | int(11) | NO | PRI | NULL | |
+------------+------------------+------+-----+---------+-------+
6 rows in set (0.05 sec)
At this point, you can drop the primary key using the syntax: alter table cursos drop primary key;. This will drop the primary key constraint only (but, not the column definition).