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{
"cells": [
{
"cell_type": "code",
"execution_count": null,
"id": "22482673",
"metadata": {},
"outputs": [],
"source": [
"import agh_db_lectures\n",
"agh_db_lectures.prepare_notebook_for_sql()"
]
},
{
"cell_type": "markdown",
"id": "83f4fffc",
"metadata": {},
"source": [
"# The \"NULL\" value"
]
},
{
"cell_type": "code",
"execution_count": null,
"id": "8ec0bb1a",
"metadata": {},
"outputs": [],
"source": [
"# Open the diagram in an image viewer for more convenience.\n",
"agh_db_lectures.nw_diagram.download_open()"
]
},
{
"cell_type": "code",
"execution_count": null,
"id": "d4c3ba6c",
"metadata": {},
"outputs": [],
"source": [
"# Connect to the (locally-hosted) database.\n",
"%sql postgresql://demo_user:demo_pwd@localhost:25432/agh_it_northwind"
]
},
{
"cell_type": "code",
"execution_count": null,
"id": "b594cc93",
"metadata": {},
"outputs": [],
"source": [
"agh_db_lectures.download_restore_nw_postgres_dump()"
]
},
{
"cell_type": "code",
"execution_count": null,
"id": "fc14542d",
"metadata": {},
"outputs": [],
"source": [
"%%sql\n",
"SELECT company_name, phone, fax\n",
"FROM customers"
]
},
{
"cell_type": "markdown",
"id": "4e1266e1",
"metadata": {},
"source": [
"_notes_\n",
"\n",
"- Theodore, what would an employee do if an customer to record in the db had no phone number?\n",
" - Give many ideas.\n",
" - What would be their pros and cons be?\n",
"- `SELECT company_name, phone, fax`\n",
"- Jupyter (well, Python) happens to have `NULL` values converted to `None`s and presented to us this way.\n",
"- Do not work:\n",
" - `WHERE fax = NULL`, and\n",
" - `WHERE fax <> NULL`.\n",
"- Do work\n",
" - `WHERE fax IS NULL`, and\n",
" - `WHERE fax IS NOT NULL`.\n",
" - Also, `WHERE fax <> '011-4988261' OR fax = '011-4988261'` won't return all table rows."
]
},
{
"cell_type": "markdown",
"id": "ad4700a9",
"metadata": {},
"source": [
"## \"NULL\" in arithmetic and comparison operations"
]
},
{
"cell_type": "code",
"execution_count": null,
"id": "c08e4ed5",
"metadata": {},
"outputs": [],
"source": [
"%%sql\n",
"SELECT NULL <> 'unknown fax number'"
]
},
{
"cell_type": "markdown",
"id": "b2619658",
"metadata": {},
"source": [
"_notes_\n",
"\n",
"- Try\n",
" - `SELECT NULL`,\n",
" - `SELECT NULL > 5`,\n",
" - `SELECT NULL <> 'unknown fax number'`,\n",
" - `SELECT NULL = NULL`, and\n",
" - `SELECT NULL IN (TRUE, FALSE, NULL)`,\n",
" - but note that a test for presence of `NULL` (or any other value) in an empty tuple (e.g., `SELECT NULL IN (SELECT 'whatever' LIMIT 0)`) would still give `FALSE`."
]
},
{
"cell_type": "markdown",
"id": "0613f611",
"metadata": {},
"source": [
"## \"NULL\" with logical operators"
]
},
{
"cell_type": "code",
"execution_count": null,
"id": "3cf39c08",
"metadata": {},
"outputs": [],
"source": [
"%%sql\n",
"SELECT NOT NULL"
]
},
{
"cell_type": "markdown",
"id": "66b696f3",
"metadata": {},
"source": [
"_notes_\n",
"\n",
"- `TRUE OR NULL`\n",
"- `FALSE AND NULL`\n",
"- `NOT`"
]
},
{
"cell_type": "markdown",
"id": "502f27dc",
"metadata": {},
"source": [
"## The \"COALESCE\" function"
]
},
{
"cell_type": "code",
"execution_count": null,
"id": "5af26eab",
"metadata": {},
"outputs": [],
"source": [
"%%sql\n",
"SELECT company_name,\n",
" CASE\n",
" WHEN fax IS NULL THEN\n",
" 'unknown'\n",
" ELSE\n",
" fax\n",
" END\n",
"FROM customers"
]
},
{
"cell_type": "markdown",
"id": "ff370664",
"metadata": {},
"source": [
"_notes_\n",
"\n",
"- Replace the `CASE` expression with `COALESCE(fax, 'unknown') AS fax`."
]
}
],
"metadata": {
"kernelspec": {
"display_name": "Python 3 (ipykernel)",
"language": "/gnu/store/q35bxk1i5blx0rcgxphhrq9xqmgha9bz-python-3.11.11/bin/python3",
"name": "python3"
},
"language_info": {
"codemirror_mode": {
"name": "ipython",
"version": 3
},
"file_extension": ".py",
"mimetype": "text/x-python",
"name": "python",
"nbconvert_exporter": "python",
"pygments_lexer": "ipython3",
"version": "3.11.11"
}
},
"nbformat": 4,
"nbformat_minor": 5
}
|
