SQL, Python & More for DuckDB - Page 8

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Pretty-printing floating-pont numbers

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

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Copying the schema of a table

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

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Deterministically shuffling data

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

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Updating CSV files in-place

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

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Query from Google Sheets

Sometimes you have data over in Google Sheets and you want quickly and easily load it into DuckDB. With read_csv and the Google Sheets 'export to csv', its just one line of code.

Execute this SQL

-- you will need to create a sharelink for your sheet.
-- note the tab id (gid) and sheet id (in the url)

from read_csv('https://docs.google.com/spreadsheets/d/{sheet_id}/export?format=csv&gid={tab_id}')
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Jacob Matson

Created 08/28/24

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Split a String and cast all elements to a desired datatypeSQL

Execute this SQL

-- Here we make use of list_transform and 
-- the fact that DuckDB knows lambda expressions
SELECT list_transform(split('01:08:22', ':'), x -> CAST (x AS INTEGER));
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Michael Simons

Created 08/22/23

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Convert CSV to Parquet and amend the schemaSQL

Execute this SQL

-- This selects only specific fields from the CSV
-- It also renames one of the fields in the target schema
COPY (SELECT col1, col2, col3 AS new_col_name
	    FROM read_csv('~/data/source.csv',AUTO_DETECT=TRUE))
  TO '~/data/target.parquet' (FORMAT 'PARQUET', CODEC 'ZSTD');
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Robin Moffatt

Created 04/25/23

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Query Parquet files and their metadataSQL

Execute this SQL

-- listing files
FROM glob('dataset/*');

-- reading from files
FROM 'dataset/*.parquet' LIMIT 100; 

-- reading parquet files metadata (min_value, max_value, null_count for each field in each file)
FROM parquet_metadata('dataset/*.parquet');

-- convert files or export tables to parquet
COPY (FROM tbl) TO 'file.parquet' (FORMAT 'parquet');
COPY 'data.csv' TO 'data.paruqet' (FORMAT 'parquet');
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Octavian Zarzu

Created 04/25/23