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Shorten insert3 example query #83

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Shorten insert3 example query #83

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Shorten example query by turning both generated and constant values i…
rngadam Apr 12, 2024
2cc014a
Update documentation to reflect getting rid of subquery
rngadam Apr 12, 2024
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4 changes: 2 additions & 2 deletions questions/updates/00030000-insert3.ex
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Original file line number Diff line number Diff line change
Expand Up @@ -6,11 +6,11 @@ Insert calculated data into a table
|QUERY|
insert into cd.facilities
(facid, name, membercost, guestcost, initialoutlay, monthlymaintenance)
select (select max(facid) from cd.facilities)+1, 'Spa', 20, 30, 100000, 800;
select max(facid)+1, 'Spa', 20, 30, 100000, 800 from cd.facilities;
|ANSWER|
<p>In the previous exercises we used <c>VALUES</c> to insert constant data into the facilities table. Here, though, we have a new requirement: a dynamically generated ID. This gives us a real quality of life improvement, as we don't have to manually work out what the current largest ID is: the SQL command does it for us.</p>

<p>Since the <c>VALUES</c> clause is only used to supply constant data, we need to replace it with a query instead. The <c>SELECT</c> statement is fairly simple: there's an inner subquery that works out the next facid based on the largest current id, and the rest is just constant data. The output of the statement is a row that we insert into the facilities table.</p>
<p>Since the <c>VALUES</c> clause is only used to supply constant data, we need to replace it with a query instead. The <c>SELECT</c> statement is fairly simple: there's a computed value that works out the next facid based on the largest current id, and the rest is just constant data. The output of the statement is a row that we insert into the facilities table.</p>

<p>While this works fine in our simple example, it's not how you would generally implement an incrementing ID in the real world. Postgres provides <c>SERIAL</c> types that are auto-filled with the next ID when you insert a row. As well as saving us effort, these types are also safer: unlike the answer given in this exercise, there's no need to worry about concurrent operations generating the same ID.</p>
|HINT|
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