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add more notes in database system
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@kekeandzeyu
kekeandzeyu committed Oct 28, 2024
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157 changes: 83 additions & 74 deletions Writerside/topics/Database-System.topic
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</chapter>
<chapter title="2 SQL Part Ⅱ" id="2-sql-part">
<chapter title="2.1 Join Queries" id="2-1-join-queries">
<p>
<format color="BlueViolet">Join:</format>
Form cross product of the
tables, output all tuples and select specific columns.
</p>
<p>
<format color="BlueViolet">Example:</format>
</p>
<p><format color="BlueViolet">Join:</format> Form cross product of the tables, output all tuples and select
specific columns.</p>
<p><format color="BlueViolet">Example:</format></p>
<p>Sailors</p>
<table style="header-row">
<tr>
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</table>
</chapter>
<chapter title="2.2 Select &amp; Where Advanced" id="2-2-select-where-advanced">
<p>
<format color="BlueViolet">Arithmetic Expressions</format>
</p>
<p><format color="BlueViolet">Arithmetic Expressions</format></p>
<code-block lang="sql" collapsible="true">
SELECT S.age, S.age-5 AS age1, 2*S.age AS age2
FROM Sailors AS S
WHERE S.sname = 'Popeye';
</code-block>
<note>
<p>Comparison is "=" not "==" !!!</p>
<p>
<format color="OrangeRed">But SQLite allows this!</format>
</p>
<p><format color="OrangeRed">But SQLite allows this!</format></p>
</note>
<p>
<format color="IndianRed">SQL Calculator</format>
</p>
<p><format color="IndianRed">SQL Calculator</format></p>
<code-block lang="sql" collapsible="true">
SELECT
log(1000) as three,
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<td>1</td>
</tr>
</table>
<p>
<format color="BlueViolet">String Comparison</format>
</p>
<p><format color="BlueViolet">String Comparison</format></p>
<compare type="left-right" first-title="Old-School SQL" second-title="Standard Regular Expressions">
<code-block lang="sql">
SELECT S.sname
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WHERE S.sname ~ 'B.*';
</code-block>
</compare>
<p>
<format color="BlueViolet">Boolean Logic vs. Set Operators</format>
</p>
<p>
<format color="IndianRed">Reserve a red
<format style="bold">or</format>
a
green boat
</format>
</p>
<p><format color="BlueViolet">Boolean Logic vs. Set Operators</format></p>
<p><format color="IndianRed">Reserve a red <format style="bold">or</format> a green boat</format></p>
<compare type="left-right" first-title="Boolean Logic" second-title="Set Operators">
<code-block lang="sql">
SELECT R.sid
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SELECT R.sid
FROM Boats B, Reserves R
WHERE R.bid = B.bid AND B.color = 'red';
\/

UNION ALL
\/

SELECT R.sid
FROM Boats B, Reserves R
WHERE R.bid = B.bid AND B.color = 'green';
</code-block>
</compare>
<p>
<format color="IndianRed">Reserve a red
<format style="bold">and</format>
a
green boat
</format>
</p>
<p><format color="IndianRed">Reserve a red <format style="bold">and</format> a green boat</format></p>
<compare type="left-right" first-title="Boolean Logic (Wrong!)" second-title="Set Operators">
<code-block lang="sql">
-- A boat cannot be red and green at the same time
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SELECT R.sid
FROM Boats B, Reserves R
WHERE R.bid = B.bid AND B.color = 'red';
\/

INTERSECT
\/

SELECT R.sid
FROM Boats B, Reserves R
WHERE R.bid = B.bid AND B.color = 'green';
</code-block>
</compare>
<p>
<format color="BlueViolet">Set Semantics</format>
</p>
<p><format color="BlueViolet">Set Semantics</format></p>
<p>Default (Think of each letter as being a tuple in a relation):</p>
<p>R = {A, A, A, A, B, B, C, D}</p>
<p>S = {A, A, B, B, B, C, E}</p>
<list type="bullet">
<li>
<p>
<format color="Fuchsia">UNION:</format>
{A, B, C, D, E}
</p>
<p><format color="Fuchsia">UNION:</format> {A, B, C, D, E}</p>
</li>
<li>
<p>
<format color="Fuchsia">INTERSECT:</format>
{A, B, C}
</p>
<p><format color="Fuchsia">INTERSECT:</format> {A, B, C}</p>
</li>
<li>
<p>
<format color="Fuchsia">EXCEPT:</format>
{D}
</p>
<p><format color="Fuchsia">EXCEPT:</format> {D}</p>
</li>
</list>
<p>
<format color="BlueViolet">"ALL": Multiset Semantics</format>
</p>
<p><format color="BlueViolet">"ALL": Multiset Semantics</format></p>
<p>R = {A, A, A, A, B, B, C, D} = {A(4), B(2), C(1), D(1)}</p>
<p>S = {A, A, B, B, B, C, E} = {A(2), B(3), C(1), E(1)}</p>
<list type="bullet">
<li>
<p>
<format color="Fuchsia">UNION ALL (sum of all cardinalities):
</format>
{A(6), B(5), C(2), D(1), E(1)}
</p>
<p><format color="Fuchsia">UNION ALL (sum of all cardinalities):</format> {A(6), B(5), C(2), D(1),
E(1)}</p>
</li>
<li>
<p>
<format color="Fuchsia">INTERSECT ALL (min of cardinalities):
</format>
<p><format color="Fuchsia">INTERSECT ALL (min of cardinalities):</format>
{A(min(4,2)), B(min(2,3)), C(min(1,1)), D(min(1,0)), E(min(0,1))}
= {A, A, B, B, C}
</p>
= {A, A, B, B, C}</p>
</li>
<li>
<p>
<format color="Fuchsia">EXCEPT ALL (subtract cardinalities):
</format>
{A(4-2), B(2-3), C(1-1), D(1-0), E(0-1)} = {A, A, D}
</p>
<p><format color="Fuchsia">EXCEPT ALL (subtract cardinalities):</format>
{A(4-2), B(2-3), C(1-1), D(1-0), E(0-1)} = {A, A, D}</p>
</li>
</list>
</chapter>
<chapter title="2.3 Nested Queries" id="2-3-nested-queries">
<list type="decimal">

<li>
<p><format color="Fuchsia">In</format></p>
<p><format color="IndianRed">Example</format></p>
<code-block lang="sql" collapsible="true">
SELECT S.sname
FROM Sailors S
WHERE S.sid IN
(SELECT R.sid
FROM Reserves R
WHERE R.bid = 102); -- Subquery
</code-block>
</li>

<li>
<p><format color="Fuchsia">Not In</format></p>
<p><format color="IndianRed">Example</format></p>
<code-block lang="sql" collapsible="true">
SELECT S.sname
FROM Sailors S
WHERE S.sid NOT IN
(SELECT R.sid
FROM Reserves R
WHERE R.bid = 103); -- Subquery
</code-block>
</li>

<li>
<p><format color="Fuchsia">Exists</format></p>
<p><format color="IndianRed">Example 1 (bit odd, but legal)</format></p>
<code-block lang="sql" collapsible="true">
SELECT S.sname
FROM Sailors S
WHERE EXISTS
(SELECT *
FROM Reserves R
WHERE R.bid = 103);
/* The query will output all the sailors if there is at least one sailor who reserves boat 103
or return nothing if there isn't */
</code-block>

<p><format color="IndianRed">Example 2 (Nested Queries with Correlation)</format></p>
<code-block lang="sql" collapsible="true">
SELECT S.sname
FROM Sailors S
WHERE EXISTS
(SELECT *
FROM Reserves R
WHERE R.bid = 103 AND R.sid = S.sid);
/* The query will output all the sailors who reserve boat 103.
* In this example, the subquery will take every tuple of the sailors and run it,
* tuple by tuple.
*/
</code-block>
</li>

</list>
</chapter>
</chapter>

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