All posts by Guilhem Bichot

New and old ways to emulate CHECK constraints, DOMAIN

Correctness of data comes in different forms. One is referential integrity, also known as foreign keys. Another is CHECK constraints. While MySQL supports foreign keys, it doesn’t support CHECK constraints. Today we will look at three ways to emulate them:

  • triggers
  • views
  • generated columns (new in MySQL 5.7)

This is also relevant to another SQL feature: DOMAIN, which, in short, is a user-defined type composed of a base type (INT, CHAR, …), a default value, and a constraint on acceptable values – the latter being some simple sort of CHECK constraint.…

Using the aggregate functions ANY, SOME, EVERY with MySQL

If you have used SQL a bit, you are certainly familiar with so-called set functions or aggregate functions COUNT, SUM, AVG, described in the manual. For example, let’s say that I am the owner of a shop and I keep track of daily sales in this table:

The first column is the number of the month, between 1 and 12, the second column is the number of the day in the month, between 1 and 31, and the third column is how much we sold on that date.…

Re-factoring some internals of prepared statements in 5.7

When the MySQL server receives a SELECT query, the query goes through several consecutive phases:

  • parsing: SQL words are recognized, the query is split into different parts following the SQL grammar rules: a list of selected expressions, a list of tables to read, a WHERE condition, …
  • resolution: the output of the parsing stage contains names of columns and names of tables.