Mysql – Get data from two tables with recursive relationships

join;MySQLPHP

I have a MySQL 5.1 DB, being used behind a PHP application to track permissions and other items that need to be issued to staff that hold various positions within an org. To track what privileges a position has, I'm using this table:

mysql> desc position_privs;
+--------------+---------+------+-----+---------+-------+
| Field        | Type    | Null | Key | Default | Extra |
+--------------+---------+------+-----+---------+-------+
| position     | int(11) | NO   |     | NULL    |       |
| priv         | int(11) | NO   |     | NULL    |       |
| date_added   | int(11) | NO   |     | 0       |       |
| date_removed | int(11) | YES  |     | NULL    |       |
+--------------+---------+------+-----+---------+-------+
4 rows in set (0.00 sec)

position refers to an id column in a positions table, and priv refers to an id column in a privs table. Some positions also inherit privileges from other positions, which may in turn inherit from yet another position, etc. I have this table for that:

mysql> desc position_inherits;
+--------------------+---------+------+-----+---------+-------+
| Field              | Type    | Null | Key | Default | Extra |
+--------------------+---------+------+-----+---------+-------+
| position           | int(11) | NO   |     | NULL    |       |
| inherit_privs_from | int(11) | NO   |     | NULL    |       |
| date_added         | int(11) | NO   |     | 0       |       |
| date_removed       | int(11) | YES  |     | NULL    |       |
+--------------------+---------+------+-----+---------+-------+
4 rows in set (0.00 sec)

If the inheritances were always only one level deep, I could do this with a simple JOIN, but I'm stumped as to handling positions that inherit from a position, which in turn inherits from another position.

Is there a convenient way of doing this using MySQL, or should I just stick to SELECT * FROM $table and doing it in the application code instead?

Best Answer

BAD NEWS

MySQL does not support recursive data or table structures. Neither does it support recursive SQL.

GOOD NEWS

There is a hope. I have written some stored functions on how to retrieve hierarchies of data

Oct 24, 2011 : Find highest level of a hierarchical field: with vs without CTEs
Dec 10, 2012 : MySQL: Tree-Hierarchical query

CAVEAT

You would have to implement this against your tables.
You also need to make sure you are entering data as a Tree and not as a Graph.
For any row in the position_inherits table that does not have a parent, simply set that row's inherit_privs_from to zero(0). That way, you have establish root positions.

Related Solutions

MySQL – Get Column from Multiple Tables

If all the tables use the MyISAM Storage Engine and have the same table structure, I have some good news for you.

You can create a single table that consumes no additional space except a .frm file and some mapping info. The key is to take advantage of the MERGE (MRG_MyISAM) Storage Engine.

Here is how you can do this:

CREATE TABLE XMerge LIKE X1;
ALTER TABLE XMerge ENGINE=MRG_MYISAM
UNION=(X1,X2,X3,X4) INSERT_METHOD=LAST;

Using this method, you can query the 4 tables at the same time like this:

SELECT ReqF FROM XMerge WHERE EmpName='John';

Was that simple, or what ???

In your case, you have 75 tables. You would do this:

CREATE TABLE XMerge LIKE X1;
ALTER TABLE XMerge ENGINE=MRG_MYISAM
UNION=(X1,X2,X3,X4,X5,X6,X7,X8,X9,
X10,X11,X12,X13,X14,X15,X16,X17,X18,X19,
X20,X21,X22,X23,X24,X25,X26,X27,X28,X29,
X30,X31,X32,X33,X34,X35,X36,X37,X38,X39,
X40,X41,X42,X43,X44,X45,X46,X47,X48,X49,
X50,X51,X52,X53,X54,X55,X56,X57,X58,X59,
X60,X61,X62,X63,X64,X65,X66,X67,X68,X69,
X70,X71,X72,X73,X74,X75) INSERT_METHOD=LAST;
SELECT ReqF FROM XMerge WHERE EmpName='John';

The beauty of this is that creating a MERGE table takes milliseconds. Just make sure every table has an index on EmpName. Better to do 75 indexed lookups that 75 full table scans. If there is no index on EmpName, you need to do this:

ALTER TABLE X1 ADD UNIQUE KEY (EmpName);
ALTER TABLE X2 ADD UNIQUE KEY (EmpName);
.
.
.
ALTER TABLE X75 ADD UNIQUE KEY (EmpName);
CREATE TABLE XMerge LIKE X1;
ALTER TABLE XMerge ENGINE=MRG_MYISAM
UNION=(X1,X2,X3,X4,X5,X6,X7,X8,X9,
X10,X11,X12,X13,X14,X15,X16,X17,X18,X19,
X20,X21,X22,X23,X24,X25,X26,X27,X28,X29,
X30,X31,X32,X33,X34,X35,X36,X37,X38,X39,
X40,X41,X42,X43,X44,X45,X46,X47,X48,X49,
X50,X51,X52,X53,X54,X55,X56,X57,X58,X59,
X60,X61,X62,X63,X64,X65,X66,X67,X68,X69,
X70,X71,X72,X73,X74,X75) INSERT_METHOD=LAST;
SELECT ReqF FROM XMerge WHERE EmpName='John';

Give it a Try !!!

Mysql – “Lost connection to MySQL server during query” error

I am running MySQL 5.7.26 on Windows and when doing just a simple query from the MySQL command line client I get that same error you describing. I found out it only occurs when the query length is exact 64 characters! Just by adding a space fixed the problem for me.
It doesn't matter if it's a select or insert/delete/update query.
I have run 66.000 queries on my server and only the queries with exact 64 characters failed.
I checked the database for corruption. But I could also reproduce same problem when making a new database and table on that server.
I could only reproduce this problem on that production server. I created a new server with same version with exact same settings as a VMWare machine but could not recreate. Used the same MySQL client.