Generic SQL – Page 2

Select values that occur multiple times

There are many cases when we want to select values that occur multiple times: customers that ordered the same product multiple times, users with the same email, and so on. We can do this with GROUP BY and HAVING.

Return missing combinations of items from different tables

How to find out which combinations of items from different tables never appear in a third table.

Check if more than N rows are returned

Counting the rows that match a WHERE condition can be slow, and may return a very high and precise number that is useful to no one. Let’s see how to just check if a query returns more than N rows.

Comments on database objects

Why and how to write comments on database, tables, and other objects on most DBMSs. How to read comments.

Make searches at the end of a text fast

Searching for a string at the end of a text column is typically slow, because no index is used. How to build a suitable index and make LIKE '%end' fast.

Find texts starting, ending or containing a given string

How to select rows where a text column starts by, ends by, or contains a string. Including slightly less trivial cases.

Aggregate data that match a condition

How to aggregate data that match a given condition. How to perform different aggregations of data that match different conditions, in a single query. Includes convenient SQLite shortcuts.

Find texts that contain HTML tags

How to select text values that contain HTML tags. How to disallow inserting texts that contain HTML tags.

Get distinct values that are stored in multiple columns

Getting a list of unique values from a column is easy. But if the same type of values are written on multiple columns and we want to eliminate duplicates, this requires a little trick.

Find rows that (don’t) contain a value from a list

How to find rows that contain, or don’t contain, a value from a list. The list may be written in the query, or stored in a table.