Let’s say the computer’s memory is built out of transistors. This electronic part can store electric charge or not, so state 1 or 0. That is the reason why it’s easier to store only 1 or 0 rather than all digits we use in our decimal system (0 1 2 3 4 5 6 7 8 9). Everything in computer is stored as number is specific way.
How is number like 142 represented in binary system consisting only of bits 1 and 0?
Computers use a group of 8 bits called byte. For example 10001110 in binary is 142 in decimal.
In dec system each position in number has different weight. Weights are powers of 10. 10^0 = 1, 10^1 = 10, 10^2 = 100, 10^3 = 1000,… So number 142 is shown here:
Sum it: 1*100 + 10*4 + 1*2 and get 142. Something similar is done also in binary.
This time the weights are powers of 2 because binary. 2^0 = 1, 2^1 = 2, 2^2 = 4, 2^3 = 8
Sum this: 1*128 + 1*8 + 1*4 + 1*2 and you get 142.
The save process can be done vice versa. Let’s convert number 180 from dec to bin.
We start with the largest power of 2 which is smaller than our number.
180 has 128 1 times, 180 – 128 = 52
52 has 64 0 times, 52 stays
52 has 32 1 times, 52 – 32 = 20
20 has 16 1 times, 20 – 16 = 4
4 has 8 0 times, 4 stays
4 has 4 1 times, 4 – 4 = 0
0 has 2 0 times, 0 stays
0 has 1 0 times, 0 stays
Then write down 1 and 0 and we get 10110100 which is 180 in bin.
We often select only a portion of out huge database. For example, we will use table from last article and we would like to select only students with bikes. Let’s do it.
SELECT id FROM students WHERE has_bicycle = true;
Examples of another where clause uses:
SELECT * FROM students WHERE surname = 'Smith';
SELECT * FROM students WHERE id = 1;
SELECT * FROM students WHERE date_of_birth < '2005/01/01';
SQL (Structured Query Language ) is language created for querying from database.
Assume you have a database. You want to insert new stuff, update it, select some stuff or maybe delete it overtime.
Easy explained, the database consists of several tables, for example table of students:
The first column is unique id of record in the table (it is not number of the row as we can delete second row and row with id 3 will be second then).
Let’s start with selection. The basic syntax is:
/* comment in sql */
/* basic syntax ended with ; */ SELECT columns FROM table;
/* examples */
/* 1 */ SELECT id FROM students;
/* 2 */ SELECT name, surname FROM students;
/* 3 */ SELECT id, name, surname, date_of_birth, has_bicycle FROM students;
/* 4 */ SELECT * FROM students;
The first query SELECT id FROM students will return whole column with ids. The last 2 queries are the same, * character means everything (every column). The result of query number 2 will be:
I asked myself a question: Would you like to build electric skateboard? And so my answer was divided to pros and cons. But first, let me tell you some keywords which run across my mind when I wanted answer my qeustion: Freedom, Expensive, Swag
- Enjoying the ride
- Get something cool for transportation
- Develop an app for it
- Add custom sensors to monitor conditions
- Waste of money
- I cannot ride everywhere, only on good roads
- It is not stable as electric scooter
Big motivation to build it was the fact that my friend wanted to build an eskate too but He was lazy and did not get enought money for realisation, cuz the price of basic eskate is around 250$. I wanted to beat him and show the eskate to him so he will be shocked. I did not tell anything about my eskate until the revelation.