interview_SQL.md

With ...

> SELECT id, color, name FROM shirts LIMIT 5

id   color     name
1    blue      Jane
2    yellow    Susan
3    green     Craig
4    yellow    Susan
5    blue      Susan

... form a SQL statement which would find

1. the number of shirts of each color
2. how many people (names) have color: blue shirts
3. how many people (names) have more than one color of shirts

(examples)

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Answers may vary but sufficient examples are ...

1. > SELECT color, count(id) FROM shirts GROUP BY color
   

2. > SELECT DISTINCT(name) FROM shirts WHERE color='blue'
   

3. > SELECT name FROM (SELECT name, COUNT(DISTINCT(color)) AS color_count FROM shirts GROUP BY name) WHERE color_count>1
   

... with an easy replication test via ...

$ pip install duckdb pandas
$ ipython
Python 3.11.2 (main, Feb 16 2023, 03:07:35) [Clang 14.0.0 (clang-1400.0.29.202)]
IPython 8.12.0 -- An enhanced Interactive Python. Type '?' for help.

# setup
In [1]: import duckdb
In [2]: import pandas as pd
In [3]: shirts = pd.DataFrame([{"id":1, "color":"blue", "name":"Jane"},{"id":2, "color":"yellow", "name":"Susan"},{"id":3, "color":"green", "name":"Craig"},{"id":4, "color":"yellow", "name":"Susan"},{"id":5, "color":"blue", "name": "Susan"}])

# loaded table
In [4]: print(duckdb.sql("SELECT * FROM shirts LIMIT 5").df())
   id   color   name
0   1    blue   Jane
1   2  yellow  Susan
2   3   green  Craig
3   4  yellow  Susan
4   5    blue  Susan

# answers
# 👉 (1)
In [5]: print(duckdb.sql("SELECT color, count(id) FROM shirts GROUP BY color").df())
    color  count(id)
0    blue          2
1  yellow          2
2   green          1

# 👉 (2)
In [12]: print(duckdb.sql("SELECT DISTINCT(name) FROM shirts WHERE color='blue'").df())
    name
0   Jane
1  Susan

# 👉 (3)
In [21]: print(duckdb.sql("SELECT name FROM (SELECT name, COUNT(DISTINCT(color)) AS color_count FROM shirts GROUP BY name) WHERE color_count>1").df())
    name
0  Susan

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