DBMS – Page 263

MongoDB $inc

January 18, 2021 by Ian

MongoDB has an $inc field update operator that allows you to increment a value by a specific amount.

You can use positive and negative values (i.e. to increment or decrement the value).

If the field doesn’t already exist, it is created with the specified value.

MongoDB $setOnInsert

January 18, 2021 by Ian

The MongoDB $setOnInsert field update operator can be used to insert a specific value during an upsert.

If an update operation results in a new document being inserted (i.e. an upsert), then the $setOnInsert operator takes effect, otherwise it has no effect.

MongoDB $max

January 17, 2021 by Ian

MongoDB has a $max operator that allows you to update the value of a field only if the specified value is greater than the current value of the field.

In other words, if the $max value is greater than the current value in the document, the $max value is used. Otherwise, the document’s value remains unchanged.

MongoDB $min

January 17, 2021 by Ian

MongoDB has a $min operator that allows you to update the value of a field only if the specified value is less than the current value of the field.

In other words, if the $min value is less than the current value in the document, the $min value is used. Otherwise, the document’s value remains unchanged.

MongoDB $pop

January 16, 2021 by Ian

In MongoDB, you can use the $pop operator to remove the first or last element from an array.

Use $pop in conjunction with a method such as update() to update the specified document with the change.

Use -1 to remove the first element, and 1 to remove the last.

MongoDB $pullAll

January 16, 2021 by Ian

In MongoDB, you can use the $pullAll operator to remove all instances of the specified values from an existing array.

Use $pullAll in conjunction with a method such as update(), updateOne(), or updateMany() to update the specified document/s with the change.

MongoDB $pull

January 15, 2021 by Ian

In MongoDB, you can use the $pull operator to remove values from an array.

Use $pull in conjunction with a method such as update() to update the specified document with the change.

Rename a Field in the Query Results in MongoDB

January 15, 2021 by Ian

Sometimes when you query a collection in MongoDB, you might not be happy with the field names. By default, the field names are simply a reflection of the field names in the actual documents.

Perhaps the field names are inconsistent, or there’s a typo. Whatever the reason, you can use the $project aggregation pipeline stage to rename a field in your query results.

In some ways, this is comparable to using an alias in SQL, as it doesn’t rename the underlying fields, it simply renames them in the query results.

MongoDB $addToSet

January 14, 2021 by Ian

In MongoDB, the $addToSet operator adds a value to an array unless the value is already present in the array.

It’s similar to the $push operator, except that $push adds the value even if the value is already present.

MongoDB $push

January 14, 2021 by Ian

In MongoDB, you can use the $push operator to append a value to an array.

You can use various modifiers to specify the value’s position in the array, the order of the elements in the array, append multiple values, etc.