We often hear terms like DDL, DML, DQL, TCL, and DCL when using relational database management systems (RDBMSs). But what exactly are they?
In this article we’ll look at what DQL stands for, and what it does.
Database Concepts
What is DML?
We often hear terms like DDL, DML, DQL, and DCL when using relational database management systems (RDBMSs). But what exactly are they?
In this article we’ll look at what DML stands for, and what it does.
What is DDL?
We often encounter terms like DDL, DML, DQL, DCL, and TCL when using relational database management systems (RDBMSs). But what do they mean?
In this article we’ll look at what DDL stands for and what it does.
Signed vs Unsigned Integers
The integer data type is probably one of the more common data types when working with database management systems (and with computing in general). The integer is a numeric data type that allows us to store certain kinds of numbers.
More specifically, an integer is the number zero (0), a positive natural number (e.g. 1, 2, 3, …) or a negative integer with a minus sign (e.g. −1, −2, −3, …). Integers contain no decimal or fractional part.
However, many computing environments distinguish between signed integers and unsigned integers.
Let’s take a look at the difference between signed integers and unsigned integers.
Database Relationships Explained
If you’re new to relational databases, and you’re trying to get your head around this concept of a “relationship” in your database, I hope this article helps.
What is a Comparison Operator?
Comparison operators are an important part of most programming languages.
Comparison operators are used to compare two expressions. The result is either true or false. It could also be unknown. This could also be represented by either 1, 0, or NULL, depending on the language. These are typically known as “Boolean expressions”.
When used with databases, comparison operators can be used inside your SQL queries to filter data to a certain criteria.
What is a CHECK Constraint?
In database terms, a CHECK constraint is a type of constraint that checks data before it enters the database.
CHECK constraints help maintain data integrity, because they prevent invalid data entering the database.
What is the Year 2038 Problem?
The Year 2038 problem (also referred to as the Y2K38 bug) refers to a problem that some computer systems might encounter when dealing with times past 2038-01-19 03:14:07.
Many computer systems, such as Unix and Unix-based systems, don’t calculate time using the Gregorian calendar. They calculate time as the number of seconds since 1 January 1970. Therefore, in these systems, time is represented as a big number (i.e. the number of seconds passed since 1970-01-01 00:00:00). This is typically referred to as Epoch time, Unix time, Unix Epoch time, or POSIX time. As I write this, Unix time is 1560913841. And as I write this next line, Unix time has incremented to 1560913879.
What is Collation in Databases?
In database systems, Collation specifies how data is sorted and compared in a database. Collation provides the sorting rules, case, and accent sensitivity properties for the data in the database.
For example, when you run a query using the ORDER BY clause, collation determines whether or not uppercase letters and lowercase letters are treated the same.
Collation is also used to determine how accents are treated, as well as character width and Japanese kana characters. Collation can also be used to distinguish between various ideographic variation selectors in certain collations (such as the Japanese_Bushu_Kakusu_140 and Japanese_XJIS_140 collations that were introduced in SQL Server 2017).
Different database management systems will provide different collation options. Depending on the DBMS, collation can be specified at the server level, the database level, the table level, and the column level. Collations can also be specified at the expression level (so you can specify which collation to use when you run a query), and at the identifier level.
“This Data” vs “These Data” – Which is Correct?
If you happen to read a lot of data-related material, you might occasionally find the word “data” being treated in different ways. In some cases you’ll see “this data is…” and in other cases “these data are…”. You might even think “they obviously made a mistake with their grammar”.
Not so fast!