sql: add beginner sql query primer

sql/learning-sql-1.md

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+# Learning SQL 1
+
+A document to help with learning how to write simple SQL queries. One can use
+the [Online Database Playground](https://pgplayground.com/) to play with the
+provided SQL examples.
+
+## SQL Queries - Lecture 1
+
+The lecture is structured such that one can copy the SQL block into the
+The click on the red `Run Queries` button and have a look at the query results.
+
+Each section covers one of following aspect:
+
+- create a table `CREATE TABLE ... ;`
+- populate it with some values `INSERT INTO ... ;`
+- select records `SELECT ... FROM ... ;`
+- filter result set with `SELECT ... FROM ... WHERE ... ;`
+- aggregate query `SELECT ... FROM ... GROUP BY ... ;`
+
+### SQL Query Structure
+
+The structure of a SQL query is:
+
+```sql
+-- get all columns from table table_name in schema schema_name
+select * from schema_name.table_name;
+
+-- same using the default schema name (which one can set)
+select * from table_name;
+```
+
+A couple of key points worth pointing out:
+
+- queries start with `SELECT`
+- queries name the source (e.g. table) from where to get the data
+- queries end with a semicolon `;`
+- queries list the column names to get, or `*` to get all columns
+- the schema name is optional as there is a default schema name
+- lines starting with two dashes `--` are comments
+
+Some more "esoteric" points:
+
+- unless in double quotes `"` the table and column names **are not case sensitive**.
+
+### Common Preparation
+
+All of the following sections use this common setup that creates and populates
+the table `employees`.
+
+> Feel free to ignore all these statements. They will be covered later.
+
+```sql
+create table employees(id int, first_name text, last_name text, hire_date date, department_id int, primary key(id));
+
+-- ***
+-- populate the table
+--
+insert into employees values (100, 'Steven'   ,'Mayer'     ,'1984-06-15', 3);
+insert into employees values (101, 'Maria'    ,'Wang'      ,'1981-09-21', 5);
+insert into employees values (102, 'Lex'      ,'De Haan'   ,'1997-01-13', 5);
+insert into employees values (103, 'Alexander', 'Bock'     ,'1992-07-08', 7);
+insert into employees values (104, 'Bruce'    ,'Ernst'     ,'1991-05-17', 7);
+```
+
+### Basic Queries
+
+```sql
+create table employees(id int, first_name text, last_name text, hire_date date, department_id int, primary key(id));
+
+-- ***
+-- populate the table
+--
+insert into employees values (100, 'Steven'   ,'Mayer'     ,'1984-06-15', 3);
+insert into employees values (101, 'Maria'    ,'Wang'      ,'1981-09-21', 5);
+insert into employees values (102, 'Lex'      ,'De Haan'   ,'1997-01-13', 5);
+insert into employees values (103, 'Alexander', 'Bock'     ,'1992-07-08', 7);
+insert into employees values (104, 'Bruce'    ,'Ernst'     ,'1991-05-17', 7);
+
+select * from employees;
+select first_name, hire_date from employees;
+select first_name, hire_date from employees order by hire_date;
+select first_name, hire_date from employees order by hire_date asc;
+select first_name, hire_date from employees order by hire_date desc;
+```
+
+1. select all records and all columns
+2. select only first name and hire date
+3. like Q2 but order by hire date (ascending)
+4. like Q3 but with explicit order direction `asc`
+5. like Q4 but with explicit descending order `desc`
+
+### Add Filter Condition - WHERE
+
+One can add a condition to a query by adding `WHERE condition` to the SQL query. Some additional rules to keep in mind:
+
+- Multiple condition can be combined with `AND` and `OR`.
+- It can be necessary to add brackets to tell which conditions go together.
+- A `AND` condition means both sides must be true.
+- A `OR` condition means that it suffices if either or both sides are true.
+
+Here is a set of sample queries to get started:
+
+```sql
+create table employees(id int, first_name text, last_name text, hire_date date, department_id int, primary key(id));
+
+-- ***
+-- populate the table
+--
+insert into employees values (100, 'Steven'   ,'Mayer'     ,'1984-06-15', 3);
+insert into employees values (101, 'Maria'    ,'Wang'      ,'1981-09-21', 5);
+insert into employees values (102, 'Lex'      ,'De Haan'   ,'1997-01-13', 5);
+insert into employees values (103, 'Alexander', 'Bock'     ,'1992-07-08', 7);
+insert into employees values (104, 'Bruce'    ,'Ernst'     ,'1991-05-17', 7);
+
+select * from employees where hire_date < '1990/01/01';
+select * from employees where hire_date < '1990/01/01' AND department_id = 5;
+select * from employees where hire_date < '1990/01/01' AND department_id = 5 ORDER BY first_name;
+```
+
+The three queries do:
+
+1. select all employees that were hired before 1990
+2. select all employees that were hired before 1990 and work in department #5
+3. same as 2 but order the result by the first name
+
+### Aggregate Query - GROUP BY
+
+One can aggregate multiple records into a summary by adding a `GROUP BY` clause.
+
+- The `group by` lists all columns by which to group, i.e. every existing distinct combination is a row in the result
+- The other columns must be aggregated by an aggregate function such as `count()`, `min()`, `max()`, and so forth.
+
+```sql
+create table employees(id int, first_name text, last_name text, hire_date date, department_id int, primary key(id));
+
+-- ***
+-- populate the table
+--
+insert into employees values (100, 'Steven'   ,'Mayer'     ,'1984-06-15', 3);
+insert into employees values (101, 'Maria'    ,'Wang'      ,'1981-09-21', 5);
+insert into employees values (102, 'Lex'      ,'De Haan'   ,'1997-01-13', 5);
+insert into employees values (103, 'Alexander', 'Bock'     ,'1992-07-08', 7);
+insert into employees values (104, 'Bruce'    ,'Ernst'     ,'1991-05-17', 7);
+
+select department_id, count(*) from employees group by department_id;
+select department_id, count(*) from employees group by department_id order by department_id;
+select department_id, count(*) from employees where hire_date < '1990/01/01' group by department_id;
+select department_id, count(*), min(hire_date) first_hire, max(hire_date) last_hire from employees group by department_id;
+```
+
+The four queries do compute:
+
+1. count the total number of employees working in each department
+2. same as Q1 but order the result set by the department ID
+3. count only hires before 1990
+4. add first and last hire date for each department
+
+## Extra Chapter - Tables
+
+### Tables - The Fundamentals
+
+As for tables, one can think of
+
+- tables like spreadsheets
+- schema are like folder in which spreadsheets are organized
+- if no schema name is specified the default schema is used
+- tables have a structure, that is column names and column types
+- tables typically have a primary key than uniquely identifies a record
+- a row in a table is called a record
+
+### Create Table SQL Statement
+
+One creates table with a `CREATE TABLE` statement:
+
+```sql
+create table employees(id int, first_name text, last_name text, birthday date, primary key(id));
+```
+
+The syntax is `create table(col1 type, col2 type, ..., primary key(colx));`:
+
+- list all the column names and their type as a comma `,` separated list
+- the list is in brackets after `create table`
+- the statement ends with a semicolon `;`
+- the last part is primary key that list all columns that together uniquely identify a record
+- it is common and a best practice to have a primary key but it is not required
+