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Malin Freeborn
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data/sql/postgresql.md
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# Setup
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Install postgres and start it as a service, then start with:
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> psql
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## Make a database as the new user postgres
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> sudo su postgres
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> [postgres] echo $HOME
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> [postgres]
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> [postgres] CREATE DATABASE dvdrental;
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## Sample Data
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Get sample data.
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> wget http://www.postgresqltutorial.com/wp-content/uploads/2019/05/dvdrental.zip
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And then get the pdf mapping the sample data:
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> wget http://www.postgresqltutorial.com/wp-content/uploads/2018/03/printable-postgresql-sample-database-diagram.pdf
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Unzip and load sample data:
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> unzip dvdrental.zip
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> sudo su postgres
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> [postgres] $ pg_restore -U postgres -d dvdrental dvdrental.tar
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> [postgres]
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# Commands
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## Basics
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List available databases.
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> \l
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You'll see a list of available databases like:
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`dnd`
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`dvdrentals`
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Then you can connect to one:
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> \c dvdrental
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And have a look at what tables it has:
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> \d dvdrental
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If it has tables such as `language`, `film_actor` and `inventory`, you can see the table's settings with:
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> \dt film_actor
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And pull back the entire table:
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> SELECT * from film_actor;
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## Various
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Connect to 231.13.48.38 with user 'bob', port 1234, database 'X'
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> psql -h 231.13.48.38 -p1234 -U bob X
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# Setup Yourself
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Make database "test" and connect to it.
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> CREATE DATABASE test;
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> \l test
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Delete database 'dogs':
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> DROP DATABASE dogs;
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Making a table has a basic form of:
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`CREATE TABLE table_name (`
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then [ column name ] + [data type ] ... (and possibly data constraints)
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`)`
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|Data Types | Meaning | Constraints |
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|:----|:----|:----|
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| BIGSERIAL | A number incrementing by one each entry | 'NOT NULL PRIMARY KEY (so it's used for relational reference) |
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| int | integer | (50) limits the table to 50, e.g. `int(50)`|
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| VARCHAR | any characters | limit, e.g.`VARCHAR(70)`|
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| TIMESTAMP | time | |
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| date | date | |
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| text | text? | |
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| tsquery | text search query | |
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| money | money | |
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| json | textual JSON data | |
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| cidr | ipv4 or 6 address | |
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| macaddr | mac address | |
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E.g.
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```
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CREATE TABLE character (
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id int,
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str int(1),
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dex int(1),
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spd int(1),
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int int(1),
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wts int(1),
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cha int(1));
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```
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See your table:
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> \d
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Look at what columns you have there:
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> \d character
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But this allows for empty characters, so...
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```
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CREATE TABLE person (
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id BIGSERIAL NOT NULL PRIMARY KEY,
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first_name VARCHAR(50) NOT NULL,
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last_name VARCHAR(50) NOT NULL,
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last_name VARCHAR(50) NOT NULL,
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gender VARCHAR(7) NOT NULL,
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date_of_birth DATE NOT NULL,
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);
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```
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Delete with
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> DROP TABLE person;
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## Inserting Data
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```
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INSERT INTO person (
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first_name,
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last_name,
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gender,
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date_of_birth)
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VALUES ('Hugi','Smith','DWARF', date '200-01-12');
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```
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## Selecting Data
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You can also mass select by choosing to insert a file. Download example data [here](https://mockaroo.com/).
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> \i /home/ghost/file.sql
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Various querries:
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> SELECT * FROM person;
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> SELECT * FROM person ORDER BY id DESC;
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> SELECT * FROM person
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## Offset, Fetch and Limit
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'Limit' is not official, but was accepted later:
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> SELECT * FROM person ORDER BY country ASC LIMIT 10;
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The official way to make a limit is 'FIRST 5 ROWS ONLY:
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> SELECT * FROM person OFFSET 5 FETCH FIRST 5 ROWS ONLY;
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> SELECT * FROM person where gender = 'Male' AND ( country_of_birth = 'Poland' OR country_of_birth = 'China');
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Miss out the first 5 result with 'OFFSET 5'.
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> SELECT p\* FROM PERSON WHERE gender = 'Female' AND country_of_birth = 'Kosovo' OFFSET 5;
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> SELECT * FROM person OFFSET 5 FETCH FIRST 7 ROW ONLY;
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## Advanced Selection
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This query takes a lot of typing:
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> SELECT * FROM person WHERE country_of_birth = 'China'
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> OR country_of_birth = 'Kosovo'
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> OR country_of_birth = 'Brazil';
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You can write the same thing with less typing:
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> SELECT *
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> FROM person
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> WHERE country_of_birth in ('China','Kosovo','Brazil');
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> SELECT * FROM person
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> WHERE date_of_birth
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BETWEEN DATE '2018-04-10' AND '2019-01-01'
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> ORDER BY date_of_birth;
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### Rough Search
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Similar words - we can find emails ending in '.com'.
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> SELECT * FROM person
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> WHERE email LIKE '%.com';
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Or any gmail address:
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> SELECT * FROM person
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> WHERE email LIKE '%@gmail.%';
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Or particular characters, where three precede 'gmail.com' and it's case insensitive:
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> SELECT * FROM person
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> WHERE email iLIKE '\_\_\_@gmail.com';
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### Groups and Aggregates
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Select all countries as a complete mess:
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> SELECT country_of_birth FROM person;
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Select countries with proper grouping:
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> SELECT country_of_birth FROM person GROUP BY country_of_birth;
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Select countries and count instances:
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> SELECT country_of_birth, COUNT(\*) FROM person GROUP BY country_of_birth ORDER BY country_of_birth;
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Also select a minimum number with 'having'. What you have must be before 'order by'.
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> SELECT country_of_birth, COUNT(\*) FROM person GROUP BY country_of_birth HAVING COUNT(\*) > 5;
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> SELECT country_of_birth, COUNT(\*) FROM person GROUP BY country_of_birth HAVING COUNT(\*) >= 10;
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Other aggregates include 'max', 'min'.
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Select most expensive car:
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> SELECT MAX(price) FROM car;
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> SELECT MIN(price) FROM car;
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> SELECT AVG(price) FROM car;
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We can stick items together for better grouping:
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> SELECT make, model, MAX(price) FROM car GROPU BY make, model;
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Select all fields from table 'car', and add a column containing another price, discounted to 90%, rounded to two decimal places.
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> SELECT id,make,model,price,ROUND(price * .9, 2) from car;
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Same thing, but take 10% of the price from the price.
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> SELECT id,make,model,price,ROUND(price - (price * .1), 2) from car;
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## Comparison
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> SELECT 10 + 2^2;
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> SELECT 10! * 2 - 3;
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... et c.
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This returns false:
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> SELECT 1 = 1;
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These return false:
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> SELECT 2<1;
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Or '1 is not equal to 1':
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> SELECT 1<>1;
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And with strings, 'G is not the same as g':
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> SELECT 'G'<>'g';
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### Car Disconts
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You want to show the discounts on various cars. You check which columns are available and select all of them:
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> \d car
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> SELECT id,make,model,price FROM car;
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## Aliases
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You can change what a column name appears as with:
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> select price AS original_price from car;
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# Null Values
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## Coalesce
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You can input a series of entries, requesting the first one which is present. Here we input three entries which are 'null', and a third which is '2', so '2' is selected:
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> SELECT COALESCE(null, null, 2) AS number;
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When selecting column 'email' from table 'person', you can input the string 'Email not provided' if there is no email provided:
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> SELECT COALESCE(email, 'Email not provided') from person;
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## Nullif
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Normally, devision by 0 produces an error:
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> SELECT 10/ 0;
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But 10 divided by 'null' produces only 'null', which is not an error.
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The 'nullif' statement takes two numbers, and returns 'null' iff the numbers are the same as each other.
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> select nullif(0,0)
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> select nullif(10,10)
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# Date
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Select date:
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> SELECT NOW()::DATE;
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> SELECT NOW()::TIME;
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or just:
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> SELECT NOW();
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More [here](postgresql.org/docs/11/datatype-datetime.html).
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2h23m
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