---
title: Database
created: 2021-01-26
updated: 2021-01-26
type: summary
tags: [tech, reference]
external: https://github.com/wahyd4/knowledge/blob/master/categories/database.md
---

# Relation
- Mysql
- Postgres
- Microsoft SQL Server
- Oracle
- Sqlite

# NoSql
- Monodb
	- Replica Set
		- Primary
		- Secondary
		- Hidden
- Redis
	- Cluster
		- Strong recommend 3 masters and 3 slaves
- Hbase
- Cassendra

# SQL

## Index
	- Index is unique but not mandatory
## join

## MYSQL

### Tips
		- datetime
			- 8 bytes
 			    `1000-01-01 00:00:00 ~ 9999-12-31 23:59:59`
			- store a absolute value
		- timestamp
			- 4 bytes `1970-01-01 08:00:01 ~ 2038-01-19 11:14:07`
			- will impacted by time zone
			- index by timestamp will be faster than datetime due to 4 bytes.

## Postgres

### Commands

```bash
\list # list databases
\c one # switch to datebase one
\dt list all the tables
```

### Create a table

```sql
CREATE TABLE pm25
(
	id SERIAL,  -- id with auto increment
	time TIMESTAMP not NULL,
	pm25 integer DEFAULT 0,
	concentration integer DEFAULT 0,
	city VARCHAR not NULL
);

```
### Import data from CSV
```sql
	COPY persons(first_name,last_name,dob,email)
	FROM 'C:\tmp\persons.csv' DELIMITER ',' CSV HEADER;
```
### Import data with SQL
```bash
psql -U postgres -W -p 5432 -d somename -h 127.0.0.1 -f  ~/somefile.sql
```

### Create user
```sql
CREATE DATABASE yourdbname;
CREATE USER youruser WITH ENCRYPTED PASSWORD 'yourpass';
GRANT ALL PRIVILEGES ON DATABASE yourdbname TO youruser;
```

### Update

```sql
UPDATE badges set type = 'onetype' where label != ''
```

### Queries

#### Query with json field

```sql
select count(*), arguments ->> 'SomeProp', arguments -> 'FirstLevel' ->> 'SecondLevel'  from jobs group by arguments ->> 'SomeProp', arguments -> 'FirstLevel' ->> 'SecondLevel'
```
### Group by

```sql
select job_ids from (
	select count(*) as c, string_agg(cast(job_id as varchar),',') as job_ids, args ->> 'ResourceID' args -> 'Measurement' ->> 'Property'
	from  jobs
	group by args ->> 'ResourceID',  args -> 'Measurement' ->> 'Property'
) as f1 where c > 1;
```
#### Group by time

To get data entries grouped by time (minute/hour/day/month)

```sql
select
  date_trunc('day', created_at), -- or hour, day, week, month, year
  count(1)
from view_records
group by 1
```

If you want to group time base on a specific timezone and the timestamp field you choose contains that information.

```sql
select
  date_trunc('day', created_at AT TIME ZONE 'Australia/Melbourne'), -- or hour, day, week, month, year
  count(1)
from view_records
group by 1
```

If the time field doesn't have timezone information, we will need to ask postgres to intercept the time to the timezone we would like to use

```sql
select
  date_trunc('day', created_at AT TIME ZONE 'Australia/Melbourne') AT TIME ZONE 'China/Beijing')
  count(1)
from view_records
group by 1
```

### Postgres trigger example

Giving we have a table called users, and the columns are:
```bash
id name age
1  tom  10
2  Dave  17
```
there is a requirement which is when some user's age turn to `18` move that person to `adults` table.
The adults table have the same fields with `users`.

The potential trigger implementation can be

```sql

CREATE OR REPLACE FUNCTION to_adults ()
	RETURNS TRIGGER
	AS $$
BEGIN
	INSERT INTO adults VALUES (new.*);
	DELETE FROM users WHERE age >= 18;
	RETURN NEW;
END;
$$
LANGUAGE plpgsql;

CREATE TRIGGER to_adults_trigger
	AFTER UPDATE ON users
	FOR EACH ROW
	WHEN (NEW.age >= 18)
	EXECUTE PROCEDURE to_adults();
```



## SQL join diagram

![sql join diagram](https://raw.githubusercontent.com/wahyd4/knowledge-mind-mapping/master/Knowledge.mindnode/resources/FC67BE77-F837-4207-B3C4-45205F7C9C40.png)

# Message Queue

# Rabbitmq

# activeMQ

# Redis