First Commit

99 Work/0 OneSec/OneSecNotes/30 Engineering Skills/Computer Science/SQL.md

@@ -0,0 +1,105 @@
+---
+title: SQL
+created_date: 2024-10-28
+updated_date: 2024-10-28
+aliases:
+tags:
+---
+# SQL
+
+SQL stands for structured query language and is used to retrieve entries from a [[Database]]. 
+Many different implementations exist such as MySQL, SQLite
+
+## Crash Course Takeaways
+```SQL
+SELECT (DISTINCT) [Column names, or * for everything]
+FROM table_name
+WHERE selection_statements;
+ORDER BY column_name (DESC)
+-- only return 20 entries
+LIMIT 20 
+```
+
+
+> [!Info] DISTINCT statement
+> This will make sure that only unique entries are returned. It is used to filter out duplicates, which is a very important step when doing statistics on a dataset. Be cautious though, depending on the SELECT statement it might filter out actual datapoints instead of duplicates (e.g. SELECT DISTINCT last_name ... would only return one of several siblings)
+
+### Filter
+- Use selection statement after the **WHERE** keyword
+- e.g. `WHERE last_name="Connor"` or `WHERE age <= 18` or `WHERE nationality in ("Swiss", "Italian", "French")`
+- `AND` , `OR`, `NOT` are logical operators
+- `WHERE name Like "%blue%"` returns any string with blue in it `(%` are placeholders)
+- `WHERE name Like "____"` returns all names with 4 characters
+- `WHERE name IS (NOT) NULL`: filter out nulls 
+### Sort
+You can sort with the Keyword `ORDER BY`. If you want to reverse the order you can add the `DESC` statement at the end.
+
+### CASE Statement
+The `CASE` statement allows to change data on the fly (e.g. grouping) without changing the underlying database entries.
+```SQL
+SELECT EmployeeName,  
+CASE  
+	WHEN EmpLevel = 1 THEN 'Data Analyst'  
+	WHEN EmpLevel = 2 THEN 'Middle Manager'  
+	WHEN EmpLevel = 3 THEN 'Senior Executive'  
+ELSE 'Unemployed'  
+END  
+  
+FROM Employees;
+```
+
+### Limit Keyword
+With the `LIMIT` keyword you can limit the number of rows the query returns. 
+
+### Functions
+- `COUNT` returns the number of entries of a query: `SELECT COUNT(*) FROM names;`
+- `SUM`: sums all entries of the query
+- `MIN`
+- `MAX`
+- `AVG`
+- 
+Functions can be executed with subgroups of the returned DB-entries. In order to achieve this the `GROUP BY column_name` statement is used (just as in the [[Pandas]] library). 
+
+```SQL
+--- The example counts players from the same team
+SELECT Team, COUNT(PlayerID)  
+FROM Players  
+GROUP BY TEAM;
+```
+
+If you want to filter a second time within the subgroups of the `GROUP BY` statement you can use the `HAVING` keyword.
+```SQL
+-- This only uses Account Types with more than 100 Accounts in the uppermost count aggregation 
+SELECT AccountType, COUNT(AccountID)  
+FROM Accounts  
+GROUP BY AccountType  
+HAVING COUNT(AccountID) > 100;
+```
+#### COALESCE to default nulls
+`COALESCE`: replace nulls with a default value
+```SQL
+SELECT AVG(COALESCE(HolidaysTaken, 0))
+FROM AnnualLeave;
+
+-- or select on of the 3 numbers as a number
+SELECT  
+	CustomerName,  
+	COALESCE(HomePhone, MobilePhone, BusinessPhone) as PhoneNumber  
+FROM Customers;
+```
+
+### Joins
+#### Inner Join (aka. join)
+Joins columns of two tables, if and only if, all datapoints exist in both tables.
+#### Left Join (aka. left outer join)
+This join includes all rows of the first table even if there is no match in the second table.
+
+### INSERT INTO
+is used to insert rows into database tables
+### UPDATE
+is used to change an existing database entry
+### DELETE
+is used to delete all entries that the queries returns: careful, this is dangerous!
+
+## Examples
+- The queries used in Obsidians Dataview are quite similar.