If the sent_at field is not there when its not set then: db.emails.count({sent_at: {$exists: false}}) If it’s there and null, or not there at all: db.emails.count({sent_at: null}) If it’s there and null: db.emails.count({sent_at: { $type: 10 }}) The Query for Null or Missing Fields section of the MongoDB manual describes how to query for null … Read more
If you don’t want to execute SQL directly, the best way is to use Any(). This is because Any() will return as soon as it finds a match. Another option is Count(), but this might need to check every row before returning. Here’s an example of how to use it: if (context.MyEntity.Any(o => o.Id == … Read more
Intuitive is …EXISTS (SELECT * .. because you really don’t care The only keyword of importance is EXISTS The choice of …EXISTS (SELECT 1 .. perpetuates the general myths and superstitions around EXISTS (eg comments on the MySQL docs). ANSI standard says “doesn’t matter” It’s more interesting to understand that EXISTS is a semi-join.
Your status-codes are also a collection, so use Contains: var allowedStatus = new[]{ “A”, “B”, “C” }; var filteredOrders = orders.Order.Where(o => allowedStatus.Contains(o.StatusCode)); or in query syntax: var filteredOrders = from order in orders.Order where allowedStatus.Contains(order.StatusCode) select order;
Simpler, shorter, faster: EXISTS. IF EXISTS (SELECT FROM people p WHERE p.person_id = my_person_id) THEN — do something END IF; The query planner can stop at the first row found – as opposed to count(), which scans all (qualifying) rows regardless. Makes a big difference with big tables. The difference is small for a condition … Read more
Create a UNIQUE constraint on your subs_email column, if one does not already exist: ALTER TABLE subs ADD UNIQUE (subs_email) Use INSERT … ON DUPLICATE KEY UPDATE: INSERT INTO subs (subs_name, subs_email, subs_birthday) VALUES (?, ?, ?) ON DUPLICATE KEY UPDATE subs_name = VALUES(subs_name), subs_birthday = VALUES(subs_birthday) You can use the VALUES(col_name) function in the … Read more
There are two relevant functions on POSIX systems: stat() and lstat(). These are used to find out whether a pathname refers to an actual object that you have permission to access, and if so, the data returned tells you what type of object it is. The difference between stat() and lstat() is that if the … Read more
You can’t return two (or multiple) columns in your subquery to do the comparison in the WHERE A_ID IN (subquery) clause – which column is it supposed to compare A_ID to? Your subquery must only return the one column needed for the comparison to the column on the other side of the IN. So the … Read more
Just in case anyone still cares: I had the same issue after copying a database directory directly using command cp -r /path/to/my/database /var/lib/mysql/new_database If you do this with a database that uses InnoDB tables, you will get this crazy ‘table does not exist’ error mentioned above. The issue is that you need the ib* files … Read more
SELECT SCHEMA_NAME FROM INFORMATION_SCHEMA.SCHEMATA WHERE SCHEMA_NAME = ‘DBName’ If you just need to know if a db exists so you won’t get an error when you try to create it, simply use (From here): CREATE DATABASE IF NOT EXISTS DBName;