Gábor Szárnyas | DuckDB Snippets

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Gábor Szárnyas

@gabor-1

‌

Updating CSV files in-placeSQL

DuckDB allows updating CSV files in-place.

Execute this SQL

COPY (FROM VALUES ('foo', 10/9), ('bar', 50/7), ('qux', 9/4) t(s, x))
TO 'example.csv';

COPY (SELECT s FROM 'example.csv') TO 'example.csv';
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Gábor Szárnyas

Created 08/13/24

‌

Specifying types in the CSV loaderSQL

We can specify types for the `read_csv` function on a per-column basis.

Execute this SQL

COPY (FROM VALUES ('foo', 10/9), ('bar', 50/7), ('qux', 9/4) t(s, x))
TO 'example.csv';

CREATE OR REPLACE TABLE example AS
    FROM read_csv('example.csv', types = {'x': 'DECIMAL(15, 3)'});
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Gábor Szárnyas

Created 08/13/24

‌

Deterministically shuffling dataSQL

Sometimes it's beneficial to shuffle data sets in a deterministic, repeatable way. We can achieve this by using the `hash` function on the `rowid` pseudocolumn.

Execute this SQL

COPY (FROM VALUES ('foo', 10/9), ('bar', 50/7), ('qux', 9/4) t(s, x))
TO 'example.csv';

CREATE OR REPLACE TABLE example AS FROM 'example.csv';
FROM example ORDER BY hash(rowid + 42);
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Gábor Szárnyas

Created 08/13/24

‌

Copying the schema of a tableSQL

We can use `LIMIT` to copy the schema of an existing table without repeating the specification of its column names/types.

Execute this SQL

COPY (FROM VALUES ('foo', 10/9), ('bar', 50/7), ('qux', 9/4) t(s, x))
TO 'example.csv';

CREATE TABLE tbl AS
    FROM example
    LIMIT 0;
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Gábor Szárnyas

Created 08/13/24

‌

Pretty-printing floating-pont numbersSQL

By casting the result to a `DECIMAL` with a fixed number of digits after the decimal point, we can pretty-print it while keeping it as a numeric value.

Execute this SQL

COPY (FROM VALUES ('foo', 10/9), ('bar', 50/7), ('qux', 9/4) t(s, x))
TO 'example.csv';

SELECT x::DECIMAL(15, 3) AS x
FROM 'example.csv';
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Gábor Szárnyas

Created 08/13/24