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.
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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;Copy code
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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.
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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';Copy code
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UK Bank HolidaysSQL
The UK government publish the UK bank holidays as a JSON object at a public endpoint. DuckDB can read the data from the endpoint, unpivot it, and expand it in a few lines!
Execute this SQL
/* division, title, date, notes, bunting */ select division, unnest(events.events, recursive:=true) from ( unpivot 'https://www.gov.uk/bank-holidays.json' on "england-and-wales", "scotland", "northern-ireland" into name division value events )Copy code
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Parquet to ExcelSQL
Editor's note: although not very intuitive, format for MS Excel XLSX files is provided by the spatial extension. While other snippets show you how to read Excel files, this shows you how to write data from a named Parquet file (or any DuckDB resultset) to Excel.
Execute this SQL
INSTALL spatial; LOAD spatial; COPY (SELECT * FROM 'source.parquet') TO 'target.XLSX' WITH (FORMAT GDAL, DRIVER 'XLSX');Copy code
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build-your-own SUMMARIZE with columns(*), unpivot, and struct.*SQL
Editor's note: Although SUMMARIZE() is fully supported in DuckDB, Hamilton shows you how to build this capability yourself using more primitive functions.
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-- C = number of columns in TBL, S = number of aggregates -- ------------------------------------------------------ -- (1) use columns(*) to produce S aggregates -- for each column as a single struct, -- producing 1 struct row and C columns with aggregate as ( from sf311 select { name: first(alias(columns(*))), type: first(typeof(columns(*))), max: max(columns(*))::varchar, min: min(columns(*))::varchar, approx_unique: approx_count_distinct(columns(*)), nulls: count(*) - count(columns(*)) } ), -- (2) unpivot to get a result with C rows and 1 column of structs columns as (unpivot aggregate on columns(*)) -- (3) explode the struct to get a result with C rows and S columns select value.* from columns;Copy code
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-- First, transform TBL (or any relation) -- into a single column of structs. with structs (from TBL select TBL) -- Then pack those structs into a list. from structs select list(TBL) as ready_to_plot;Copy code
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--Below SQL to generate MD5 of a string select md5('test')Copy code
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-- list and array_agg take in their own ORDER BY clause, so that you -- can sort the aggregate. The statements order by cannot be used -- as the columns that are used for sorting then need to be a grouping -- key and cannot be used in the aggregate SELECT name, -- Order the aggregated list by another column from line_items list(item_name ORDER BY pos ASC) items FROM orders JOIN line_items ON order_id = id GROUP BY ALL -- Order by grouping keys is ofc possible ORDER BY name;Copy code
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