How do I build a "if-else" SQL query in an INSERT? - sql

Question

I'm trying to copy columns from one SQLite table to another but replacing a boolean (0 or 1) with an enumeration (0 to 9). A given item could be in the table twice - once with each boolean setting. I want to copy the record with a true boolean preferentially, and if one doesn't exist then I want to copy the record with a false boolean.
Is there a way to do that in a single query?
I've included my current approach below, but that requires multiple queries to be run over the same set of tables so would be less efficient than just one query.
// Table schemas
Table1(Col1 text, Col2 text, Col3_bool int, Col4 int, Primary Key(Col1, Col2, Col3))
Table2(Col1 text, Col2 text, Col3_enum int, Col4 int, Primary Key(Col1, Col2, Col3))
// Pseudo code
for (int i = 0; i < 10; ++i)
{
insert into Table2 select Col1, Col2, i, Col4 from Table1 where Col3_bool = 0;
insert or replace into Table2 select Col1, Col2, i, Col4 from Table1 where Col3_bool = 1;
}

Answer 1

As long as you use a single transaction for all queries, it is unlikely that any performance difference is noticeable.
For each Col1/Col2 combination, you want the maximum Col3 value:
SELECT Col1, Col2, MAX(Col3_bool)
FROM Table1
GROUP BY Col1, Col2
You can then use these values to get the desired rows:
SELECT Table1.Col1,
Table1.Col2,
Table1.Col3_bool,
Table1.Col4
FROM Table1
JOIN (SELECT Col1,
Col2,
MAX(Col3_bool) AS Col3_bool
FROM Table1
GROUP BY Col1,
Col2
) USING (Col1, Col2, Col3_bool)
To get ten rows for each of these rows, do a cross join with another table with ten values:
INSERT INTO Table2
SELECT Table1.Col1,
Table1.Col2,
enums.i,
Table1.Col4
FROM Table1
JOIN (SELECT Col1,
Col2,
MAX(Col3_bool) AS Col3_bool
FROM Table1
GROUP BY Col1,
Col2
) USING (Col1, Col2, Col3_bool)
JOIN (SELECT 0 AS i UNION ALL
SELECT 1 UNION ALL
SELECT 2 UNION ALL
SELECT 3 UNION ALL
SELECT 4 UNION ALL
SELECT 5 UNION ALL
SELECT 6 UNION ALL
SELECT 7 UNION ALL
SELECT 8 UNION ALL
SELECT 9) AS enums