MySQL Multiple table locks

MySQL

I have a small curiosity about MySQL table locks.

Say I want to lock two tables. I execute this command:

LOCK TABLES table1 WRITE, table2 WRITE

And then I checked if the tables have indeed been locked by executing:

SHOW OPEN TABLES IN mydatabase WHERE In_use > 0

I have noticed tho that if I run two lock commands sequentitally for example:

LOCK TABLES table1 WRITE
LOCK TABLES table2 WRITE

And then check which tables are locked using the same command only table2 is marked as locked. Why is this so?

Thanks

Best Answer

LOCK TABLES is not transaction-safe and implicitly commits any active transaction before attempting to lock the tables.

So, in the first case, you have one transaction which hold 2 tables locked, in the second only one, because LOCK TABLES table1 WRITE had been commited

Related Solutions

Mysql – concatenate multiple MySQL rows into one field

You can use GROUP_CONCAT:

SELECT person_id,
   GROUP_CONCAT(hobbies SEPARATOR ', ')
FROM peoples_hobbies
GROUP BY person_id;

As Ludwig stated in his comment, you can add the DISTINCT operator to avoid duplicates:

SELECT person_id,
   GROUP_CONCAT(DISTINCT hobbies SEPARATOR ', ')
FROM peoples_hobbies
GROUP BY person_id;

As Jan stated in their comment, you can also sort the values before imploding it using ORDER BY:

SELECT person_id, 
       GROUP_CONCAT(hobbies ORDER BY hobbies ASC SEPARATOR ', ')
FROM peoples_hobbies
GROUP BY person_id;

As Dag stated in his comment, there is a 1024 byte limit on the result. To solve this, run this query before your query:

SET group_concat_max_len = 2048;

Of course, you can change 2048 according to your needs. To calculate and assign the value:

SET group_concat_max_len = CAST(
                     (SELECT SUM(LENGTH(hobbies)) + COUNT(*) * LENGTH(', ')
                           FROM peoples_hobbies
                           GROUP BY person_id) AS UNSIGNED);

Mysql – Which MySQL data type to use for storing boolean values

For MySQL 5.0.3 and higher, you can use BIT. The manual says:

As of MySQL 5.0.3, the BIT data type is used to store bit-field values. A type of BIT(M) enables storage of M-bit values. M can range from 1 to 64.

Otherwise, according to the MySQL manual you can use BOOL or BOOLEAN, which are at the moment aliases of tinyint(1):

Bool, Boolean: These types are synonyms for TINYINT(1). A value of zero is considered false. Non-zero values are considered true.

MySQL also states that:

We intend to implement full boolean type handling, in accordance with standard SQL, in a future MySQL release.

References: http://dev.mysql.com/doc/refman/5.5/en/numeric-type-overview.html

Best Answer

Related Solutions

Mysql – concatenate multiple MySQL rows into one field

Mysql – Which MySQL data type to use for storing boolean values

Related Topic