One of my friend created a table in mySQl, how can I check whether he applied a particular constraint on a column name or not.
In my case, I want to verify whether he applied a CHECK constraint on the column CHG_HOUR or not? The query DESC JOB does not help me with the 'CHECK' constraint.
mysql> desc JOB;
+--------------+---------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+--------------+---------------+------+-----+---------+-------+
| JOB_CODE | char(3) | NO | PRI | NULL | |
| JOB_CLASS | char(30) | YES | | NA | |
| CHG_HOUR | decimal(12,2) | YES | | NULL | |
| OVERTIME_CHG | int(11) | NO | | NULL | |
+--------------+---------------+------+-----+---------+-------+
4 rows in set (0.00 sec)
Short answer: he didn't.
MySQL doesn't support check constraints until 8.0.16 which, at the time of writing, does not seen to be even released.
Related
Apologies if this is a stupid question, but this is my first time using MySQL and I can't seem to get this to work:
I have this table:
+------------------------------+------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+------------------------------+------------------+------+-----+---------+----------------+
| id | int(10) unsigned | NO | PRI | NULL | auto_increment |
| policy_name | varchar(255) | NO | UNI | | |
| virus_lover | char(1) | YES | | NULL | |
| spam_lover | char(1) | YES | | NULL | |
| unchecked_lover | char(1) | YES | | NULL | |
| banned_files_lover | char(1) | YES | | NULL | |
| bad_header_lover | char(1) | YES | | NULL | |
| bypass_virus_checks | char(1) | YES | | NULL | |
and I would like to change virus_lover to NO
I have tried using
update policy set Null='N' where Field='virus_lover';
But this gives me a syntax error response. I have checked online and everyone seems to be suggesting this exact same command that doesn't seem to work for me.
Any help would be greatly appreciated.
MySQL Ver 14.14 Distrib 5.7.25
The correct way to modify column attributes is with the ALTER TABLE command:
ALTER TABLE policy MODIFY virus_lover CHAR(1) NOT NULL
Demo on dbfiddle
Since NULL is a reserved keyword and it's being used as a column name in the table, it must be quoted with backticks or brackets:
update policy set `Null`='N' where Field='virus_lover';
or
update policy set [Null]='N' where Field='virus_lover';
Changing the column name to a non-reserved keyword instead of NULL would make things easier as well.
When trying to describe a table I get a table with missing information (see first table); what would be the best way to update that table in a way that looks like the second one? My SQL background is not so strong, so I'd love to hear ideas on how to do this
First table (how it is actually)
+-----------+------------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-----------+------------------+------+-----+---------+-------+
| config_id | int(10) unsigned | NO | | 0 | |
| scope | varchar(8) | NO | | default | |
| scope_id | int(11) | NO | | 0 | |
| path | varchar(255) | NO | | general | |
| value | text | YES | | NULL | |
+-----------+------------------+------+-----+---------+-------+
Second table (how it should be)
+-----------+------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+-----------+------------------+------+-----+---------+----------------+
| config_id | int(10) unsigned | NO | PRI | NULL | auto_increment |
| scope | varchar(8) | NO | MUL | default | |
| scope_id | int(11) | NO | | 0 | |
| path | varchar(255) | NO | | general | |
| value | text | YES | | NULL | |
+-----------+------------------+------+-----+---------+----------------+
I see following differences between the two tables :
column config_id should be auto_increment and also primary key of the table
a (non-unique) index is missing on column scope
The following statement should change the table as required :
ALTER TABLE mytable
MODIFY COLUMN config_id INT auto_increment,
ADD PRIMARY KEY (config_id),
ADD INDEX idx_scope(scope)
;
PS : DEFAULT NULL does not make sense for config_id : since it is a primary key, your RDBMS will never allow it to be set to NULL.
Please note that this answer is based on the information you provided only. Running this statement will not necessarily make the table structures strictly equivalent, since there could be other differences that can not be seen in the representation that you provided. You can get a complete DDL statement describing the table using the SHOW CREATE TABLE mytable syntax.
I have two databases test & test2. Both have the same tables(employees & salaries) and both have the same records. test2 database uses a foreign key and test database doesn't.
test structure
test.employees
+--------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+--------+-------------+------+-----+---------+-------+
| emp_id | int(11) | NO | PRI | NULL | |
| name | varchar(30) | YES | | NULL | |
+--------+-------------+------+-----+---------+-------+
test.salaries
+--------+---------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+--------+---------+------+-----+---------+----------------+
| id | int(11) | NO | PRI | NULL | auto_increment |
| salary | int(11) | YES | | NULL | |
| emp_id | int(11) | NO | | NULL | |
+--------+---------+------+-----+---------+----------------+
test2 structure
test2.employees
+--------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+--------+-------------+------+-----+---------+-------+
| emp_id | int(11) | NO | PRI | NULL | |
| name | varchar(30) | YES | | NULL | |
+--------+-------------+------+-----+---------+-------+
test2.salaries
+--------+---------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+--------+---------+------+-----+---------+----------------+
| id | int(11) | NO | PRI | NULL | auto_increment |
| salary | int(11) | YES | | NULL | |
| emp_id | int(11) | NO | MUL | NULL | |
+--------+---------+------+-----+---------+----------------+
I run the same join query on both databases
select * from employees inner join salaries on employees.emp_id=salaries.emp_id;
This is the output i get from test database which doesn't contain a foreign key
2844047 rows in set (3.25 sec)
This is the output i get from test2 database which contains a foreign key
2844047 rows in set (17.21 sec)
So does the foreign key slow down the join query?
Your empirical evidence suggests that in at least one case it does. So, if we believe your numbers, the answer is clearly "yes" -- and I assume you have ruled out other potential causes such as locks on the table or resource competition (actually the difference is pretty big). I presume that you want to know why.
In most databases, declaring a foreign key is about relational integrity. It would have no effect on the optimization of queries. The join conditions in the query would redundantly cover the same information.
However, MySQL does a bit more when a foreign key is declared. A foreign key declaration automatically creates an index on the columns being used. This is not standard behavior -- I'm not even sure if any other database does this.
Normally, an index would benefit performance. In this case, the optimizer has more choices on how to approach the query. For whatever reason, it is using a substandard execution plan.
You should be able to look at the explain plans and see a difference. The issue is that the optimizer has chosen the wrong plan. I would say that this is uncommon and should not dissuade you from using proper foreign key declarations in your databases.
I am posting this thread in order to have some advices regarding the performance of my SQL query.
I have actually 2 tables, one which called HGVS_SNP with about 44657169 rows and another on run table which has an average of 2000 rows.
When I try to update field Comment of my run table it takes lot's of time to perform the query. I was wondering if there is any method to increase my SQL query.
Structure of HGVS_SNP Table:
+-----------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-----------+-------------+------+-----+---------+-------+
| snp_id | int(11) | YES | MUL | NULL | |
| hgvs_name | text | YES | | NULL | |
| source | varchar(8) | NO | | NULL | |
| upd_time | varchar(32) | NO | | NULL | |
+-----------+-------------+------+-----+---------+-------+
My run table has the following structure:
+----------------------+--------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+----------------------+--------------+------+-----+---------+-------+
| ID | varchar(7) | YES | | NULL | |
| Reference | varchar(7) | YES | MUL | NULL | |
| HGVSvar2 | varchar(120) | YES | MUL | NULL | |
| Comment | varchar(120) | YES | | NULL | |
| Compute | varchar(20) | YES | | NULL | |
+----------------------+--------------+------+-----+---------+-------+
Here's my query:
UPDATE run
INNER JOIN SNP_HGVS
ON run.HGVSvar2=SNP_HGVS.hgvs_name
SET run.Comment=concat('rs',SNP_HGVS.snp_id) WHERE run.Compute not like 'tron'
I`m guessing since you JOIN a text column with a VARCHAR(120) column that you don`t really need a text column. Make it a VARCHAR so you can index it
ALTER TABLE `HGVS_SNP` modify hgvs_name VARCHAR(120);
ALTER TABLE `HGVS_SNP` ADD KEY idx_hgvs_name (hgvs_name);
This will take a while on large tables
Now your JOIN should be much faster,also add an index on compute column
ALTER TABLE `run` ADD KEY idx_compute (compute);
And the LIKE is unnecessary,change it to
WHERE run.Compute != 'tron'
I'm trying to setup a mysql trigger. So lets assume the table is like this
mysql> explain users;
+------------+-------------+------+-----+-------------------+-----------------------------+
| Field | Type | Null | Key | Default | Extra |
+------------+-------------+------+-----+-------------------+-----------------------------+
| id | int(11) | NO | PRI | NULL | auto_increment |
| name | varchar(30) | YES | | NULL | |
| age | int(11) | YES | | NULL | |
| purge_date | timestamp | NO | | CURRENT_TIMESTAMP | on update CURRENT_TIMESTAMP |
+------------+-------------+------+-----+-------------------+-----------------------------+
and
mysql> explain users_details;
+-------+-------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+----------------+
| id | int(11) | NO | PRI | NULL | auto_increment |
| name | varchar(30) | YES | | NULL | |
| age | int(11) | YES | | NULL | |
+-------+-------------+------+-----+---------+----------------+
3 rows in set (0.00 sec)
so the trigger i have setup will create a purge_date of NOW on the users table when something is removed from the users_details table. This is fine.
The issue is that sometimes the data for the users_details table comes through incorrectly and then the users update it so then i need to remove the purge_date if they re-insert the data. Now I have made this trigger
CREATE TRIGGER purge_fix AFTER INSERT ON users_details
FOR EACH ROW
BEGIN
UPDATE users
SET purge_date= '0000-00-00 00:00:00'
WHERE users.name = NEW.name;
END;
Which works fine but in some situations there will not be a link between the users_details table and the users table (it will be created at a later stage) so my question is, as the trigger will fail in some situations (which is 100% fine by me), will I be breaking something having the trigger failing?