Is it possible to have a table where one of the columns is divided at a different position than the rest of the columns?
An example:
+---+---+---+
| | | |
| +---+---+
| | | |
+---+ | |
| | | |
+---+---+---+
You can do this via the rowspan property.
A table cell with rowspan will occupy the current row and that many rows below it. You must remember to omit the corresponding table cells on subsequent rows.
http://jsfiddle.net/BmJaw/
Related
Tough to say it in word. So here's an example
| Variable_name | Value |
+--------------------------+------------+
| Aborted_clients | 0 |
| Aborted_connects | 0 |
| Bytes_received | 155372598 |
if there's a table like this, is there where to see exact same table but column name changed without knowing the column names?
So for example, let's say I want to see above table with different column names.
If I know the column names, it could be done by some kind of like
Select VARIABLE_NAME as var_name, VALUE as val .....
which will give us
| var_name | val |
+--------------------------+------------+
| Aborted_clients | 0 |
| Aborted_connects | 0 |
| Bytes_received | 155372598 |
I'm wondering if this could be done without knowing the column names.
I have the following (simplified) data structure:
Property Product
----------- n <----> 1 -------
product_id manufacturer
type name
value
Where the products table might look like this
| name |
|--------------|
| iPad 2 |
| iPhone 6 |
and the associated properties might be
| product_id | type | value |
|----------------------------------|
| 1 | RAM | 16GB |
| 1 | CPU | A11 |
| 1 | Screen Size | 10" |
| 2 | Cellular | yes |
| 2 | Screen Size | 5.5" |
Is it possible, in MySQL, to create a constraint, so that every product can have each property type at most once (e.g., a product can't have multiple Screen Sizes associated. As far as I could understand the MySQL docs, constraints only work on one relation, but not across relations.
I realized the solution immediately after posting this question. The constraint can be placed on the properties relation, so that each pair of product_id, type can only exist once.
I have a thousand of records in my database mysql and I use pagination to retrieve just 10 results.
When i add a order by in my query it slow down but when i omit it the query run very fast.
I know that the problem come from that the query load whole results, sort them and after that it get the 10 records.
I don't use index because the column use for order is a PK and i think if i'm not wrong in mysql a index is created automatically on every primary key
Why the index on my PK which is the column I'm ordering.
not used ?
Is there any alternative solution to perform sorting without load all the data ?
How to add new inserted data at the first row of tables and not at the end of the table ?
My sql query
select distinct ...... order by appeloffre0_.ID_APPEL_OFFRE desc limit 10
and my indexes
mysql> show index from appel_offre;
+-------------+------------+--------------------+--------------+---------------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-------------+------------+--------------------+--------------+---------------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| appel_offre | 0 | PRIMARY | 1 | ID_APPEL_OFFRE | A | 13691 | NULL | NULL | | BTREE | | |
| appel_offre | 1 | appel_offre_ibfk_1 | 1 | ID_APPEL_OFFRE_MERE | A | 2 | NULL | NULL | YES | BTREE | | |
| appel_offre | 1 | appel_offre_ibfk_2 | 1 | ID_ACHETEUR | A | 2 | NULL | NULL | | BTREE | | |
| appel_offre | 1 | appel_offre_ibfk_3 | 1 | USER_SAISIE | A | 2 | NULL | NULL | YES | BTREE | | |
| appel_offre | 1 | appel_offre_ibfk_4 | 1 | USER_VALIDATION | A | 4 | NULL | NULL | YES | BTREE | | |
| appel_offre | 1 | ao_fk_3 | 1 | TYPE_MARCHE | A | 2 | NULL | NULL | YES | BTREE | | |
| appel_offre | 1 | ao_fk_5 | 1 | USER_CONTROLE | A | 2 | NULL | NULL | YES | BTREE | | |
+-------------+------------+--------------------+--------------+---------------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
7 rows in set (0.03 sec)
no index was chosen in explain cmd:
+----+-------------+---------------+--------+-------------------------------------+--------------------+---------+----------------
| id | select_type | table | type | possible_keys | key | key_len | ref
+----+-------------+---------------+--------+-------------------------------------+--------------------+---------+----------------
| 1 | SIMPLE | appeloffre0_ | ALL | NULL | NULL | NULL | NULL
UPDATE SOLUTION
the problem was from distinct when i delete it the query finnaly use the index.
Because you already use an index on "USER_VALIDATION", MySQL won't use the ID index instead.
Try rebuilding the USER_VALIDATION index to include the ID too:
CREATE UNIQUE INDEX appel_offre_ibfk_4 ON appel_offre (USER_VALIDATION, ID);
Update
Log all Hibernate queries, extract the slow query and use EXPLAIN in a db console to understand what execution plan MySQL selects for this query. It may be possible for the db to use a FULL TABLE SCAN even when you have an index, because the index is too large to fit into memory. Try giving it a HINT as explained in this post.
According to MySQL ORDER BY optimization documentation you should:
To increase ORDER BY speed, check whether you can get MySQL to use indexes rather than an extra sorting phase. If this is not possible, you can try the following strategies:
• Increase the sort_buffer_size variable value.
• Increase the read_rnd_buffer_size variable value.
• Use less RAM per row by declaring columns only as large as they need
to be to hold the values stored in them. For example, CHAR(16) is
better than CHAR(200) if values never exceed 16 characters.
• Change the tmpdir system variable to point to a dedicated file
system with large amounts of free space. The variable value can list
several paths that are used in round-robin fashion; you can use this
feature to spread the load across several directories. Paths should be
separated by colon characters (“:”) on Unix and semicolon characters
(“;”) on Windows, NetWare, and OS/2. The paths should name directories
in file systems located on different physical disks, not different
partitions on the same disk.
Also make sure DISTINCT doesn't overrule your index. Try removing it and see if it helps.
Add an index to the column by which you are ordering.
You can't add rows to the beginning of the table, just like you can't add rows to the end of the table. Database tables are multisets. Multisets are by definition unordered collections. The notion of a first element or a last element makes no sense for multisets.
Can I have a table with few rows in which the second row to start under the second column?
Something like this:
+-----------+----------------+
| | |
| | |
+-----------+----------------+
| |
| |
+-----------+----------------+
| | |
| | |
+-----------+----------------+
No, you cannot. But you can make it look like so by using right border and background colors.
Working example: http://jsfiddle.net/gkuQD/
You could have:
<tr><td>Content1</td><td>Content2</td>
<td> </td><td>Content3</td>
<td>Content4</td><td>Content5</td></tr>
This may be what you're looking for.
I have a table like this:
+-------+---------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+---------+------+-----+---------+-------+
| v1 | int(11) | YES | MUL | NULL | |
| v2 | int(11) | YES | MUL | NULL | |
+-------+---------+------+-----+---------+-------+
There is a tremendous amount of duplication in this table. For instance, elements like the following:
+------+------+
| v1 | v2 |
+------+------+
| 1 | 2 |
| 1 | 3 |
| 1 | 4 |
| 1 | 5 |
| 1 | 6 |
| 1 | 7 |
| 1 | 8 |
| 1 | 9 |
| 2 | 1 |
| 4 | 1 |
| 5 | 1 |
| 6 | 1 |
| 7 | 1 |
| 8 | 1 |
| 9 | 1 |
+------+------+
The table is large with 1540000 entries. To remove the redundant entries (i.e. to get a table having only (1,9) and no (9,1) entries), I was thinking of doing it with a subquery but is there a better way of doing this?
Actually, #Mark's approach will work too. I just figured out another way of doing it and was wondering if I can some feedback on this as well. I tested it and it seems to work fast.
SELECT v1,v2 FROM table WHERE v1<v2 UNION SELECT v2,v1 FROM table WHERE v1>v2;
In the case where this is right, you can always create a new table:
CREATE TABLE newtable AS SELECT v1,v2 FROM edges WHERE v1<v2 UNION SELECT v2,v1 FROM edges WHERE v1>v2;
Warning: these commands modify your database. Make sure you have a backup copy so that you can restore the data again if necessary.
You can add the requirement that v1 must be less than v2 which will cut your storage requirement roughly in half. You can make sure all the rows in the database satisfy this condition and reorder those that don't and delete one of the rows when you have both.
This query will insert any missing rows where you have for example (5, 1) but not (1, 5):
INSERT INTO table1
SELECT T1.v2, T1.v1
FROM table1 T1
LEFT JOIN table1 T2
ON T1.v1 = T2.v2 AND T1.v2 = T2.v1
WHERE T1.v1 > T1.v2 AND T2.v1 IS NULL
Then this query deletes the rows you don't want, like (5, 1):
DELETE table1 WHERE v1 > v2
You might need to change other places in your code that were programmed before this constraint was added.