SQL has been around for decades and supports a many billion dollar market. However, many people still struggle with just how to pronounce the term SQL. Is it “S.Q.L” [ˈɛs kjuː ˈɛl] or is it “sequel” [ˈsiːkwəl]?SQL… Where it all startedLet’s start at the beginning.Relational databases came into existence with E.F. Codd’s 1970 publication “A Relational Model of Data for Large Shared Data Banks.” While Codd’s ideas were remarkable for the time, in San Jose, California, two colleagues named
The concept ofmaterialized views (MVs)is almost 15 years old; Oracle first introduced these views in the 8i version of its DBMS. However, some well known DB vendors (like MySQL) still don’t support MVs or have added this functionality only quite recently (it’s available in PostgreSQL since version 9.3, which was released just a year ago). In this article I’ll try to give you some tips about when you should use MVs in OLTP systems.
Generally, we don’t limit query results. However, when we only care about the first few rows or to implement table pagination, limiting query results is just what we need. Database vendors provide us with such functionality; most of them in their own distinct way.ExampleLet’s take a look at the 2014 Sochi Olympics Men’s Normal Hill Individual ski jumping results in theskijump_resultstable. There is no index on theskijump_resultstable. The following queries are examples of a Top-N and pagination query, which I will use in the following parts of the article.
When you create a foreign key in your database, you can specify what happens upon delete of the parent row. There are usually four possibilities:ON DELETE SET NULLON DELETE CASCADEON DELETE NO ACTIONON DELETE RESTRICTToday we’ll investigate the subtle difference between the last two options.In Some Databases There Is No Difference at AllIn Oracle, there is no RESTRICT keyword . The only option is NO ACTION. In MySQL, there is
group_concat in MySQLMySQL has a very handy function which concatenates strings from a group into one string. For example, let’s take a look at thechildrentable with data about parents’ and children’s names.To get the names of children of each person as a comma-separated string, you use thegroup_concatfunctions as follows:The result:To make sure the names of the children in each string arealphabetically orderedand to use semicolon „;” as aseparator, use this query:
A real-life example:Let’s assume that we have a system which stores data fordistributorswho sell products manufactured by some company. Each distributor receives points for selling products and may redeem a specified number of points to obtain a discount or extra bonus. Points are calculated byan external systemand updatedvery frequently– after processing of each order placed by the distributor himself, a member of the distributor group, or a customer assigned to the distributor. The distributor is identified using a distributor number. We are using a DB2 database.