如何将 Parquet 文件转换为 CSV 或 JSON？

将文件从 Parquet 转换为 CSV 或 JSON

你可以使用 clickhouse-local 在 ClickHouse 支持的任意输入和输出格式之间转换文件 (支持超过 70 种不同格式！) 。本文将把 S3 中的一个 Parquet 文件转换为 CSV 和 JSON 文件。

我们先从基础讲起。ClickHouse 提供了一组表函数，可从文件、数据库和其他资源中读取数据，并将其转换为表。为了演示，假设我们在 S3 中有一个 Parquet 文件。我们将使用 s3 表函数来读取它 (ClickHouse 会根据文件名判断它是 Parquet 文件) 。

不过首先，先下载 clickhouse 二进制文件：

curl https://clickhouse.com/ | sh

使用表函数访问数据

让我们对 s3 表函数创建的结果表执行 DESCRIBE，验证是否能够读取该文件：

./clickhouse local -q "DESCRIBE s3('https://datasets-documentation.s3.eu-west-3.amazonaws.com/house_parquet/house_0.parquet')"

这个文件包含英国已售房产的房价数据。返回结果如下所示：

price	Nullable(Int64)
date	Nullable(UInt16)
postcode1	Nullable(String)
postcode2	Nullable(String)
type	Nullable(String)
is_new	Nullable(UInt8)
duration	Nullable(String)
addr1	Nullable(String)
addr2	Nullable(String)
street	Nullable(String)
locality	Nullable(String)
town	Nullable(String)
district	Nullable(String)
county	Nullable(String)

你可以对这些数据执行任何所需的查询。例如，来看看哪些城镇的房屋平均价格最高：

./clickhouse local -q "SELECT
   town,
   avg(price) AS avg_price
FROM s3('https://datasets-documentation.s3.eu-west-3.amazonaws.com/house_parquet/house_0.parquet')
GROUP BY town
ORDER BY avg_price DESC
LIMIT 10"

响应如下：

GATWICK	16818750
CHALFONT ST GILES	938090.0985915493
VIRGINIA WATER	789301.1320224719
COBHAM	699874.7111622555
BEACONSFIELD	677247.5483146068
ESHER	616004.6888297872
KESTON	607585.8597560975
GERRARDS CROSS	566330.2959086584
ASCOT	551491.2975753123
WEYBRIDGE	548974.828692494

将 Parquet 文件转换为 CSV

你可以将任何 SQL 查询的结果写入文件。让我们从 S3 中的 Parquet 文件提取所有列，并将输出写入一个新的 CSV 文件。由于输出文件以 .csv 结尾，ClickHouse 会自动使用 CSV 输出格式：

./clickhouse local -q "SELECT *
FROM s3('https://datasets-documentation.s3.eu-west-3.amazonaws.com/house_parquet/house_0.parquet')
INTO OUTFILE 'house_prices.csv'"

让我们来验证一下是否已成功：

$ tail house_prices.csv
70000,10508,"YO8","9XN","detached",0,"freehold","7","","POPPY CLOSE","SELBY","SELBY","SELBY","NORTH YORKSHIRE"
130000,14274,"YO8","9XP","detached",0,"freehold","10","","HEATHER CLOSE","","SELBY","SELBY","NORTH YORKSHIRE"
150000,18180,"YO8","9XP","detached",0,"freehold","11","","HEATHER CLOSE","","SELBY","SELBY","NORTH YORKSHIRE"
157000,18088,"YO8","9XP","detached",0,"freehold","12","","HEATHER CLOSE","","SELBY","SELBY","NORTH YORKSHIRE"
134000,17333,"YO8","9XP","semi-detached",0,"freehold","16","","HEATHER CLOSE","","SELBY","SELBY","NORTH YORKSHIRE"
250000,13405,"YO8","9YA","detached",0,"freehold","6","","YORKDALE COURT","HAMBLETON","SELBY","SELBY","NORTH YORKSHIRE"
59500,11166,"YO8","9YB","semi-detached",0,"freehold","4","","YORKDALE DRIVE","HAMBLETON","SELBY","SELBY","NORTH YORKSHIRE"
142500,17648,"YO8","9YB","semi-detached",0,"freehold","4A","","YORKDALE DRIVE","HAMBLETON","SELBY","SELBY","NORTH YORKSHIRE"
230000,15125,"YO8","9YD","detached",0,"freehold","1","","ONE ACRE GARTH","HAMBLETON","SELBY","SELBY","NORTH YORKSHIRE"
250000,15950,"YO8","9YD","detached",0,"freehold","3","","ONE ACRE GARTH","HAMBLETON","SELBY","SELBY","NORTH YORKSHIRE"

将 Parquet 文件转换为 JSON

要将 Parquet 文件转换为 JSON，只需更改输出文件的扩展名：

./clickhouse local -q "SELECT *
FROM s3('https://datasets-documentation.s3.eu-west-3.amazonaws.com/house_parquet/house_0.parquet')
INTO OUTFILE 'house_prices.ndjson'"

我们来验证一下是否已生效：

 $ tail house_prices.ndjson
{"price":"70000","date":10508,"postcode1":"YO8","postcode2":"9XN","type":"detached","is_new":0,"duration":"freehold","addr1":"7","addr2":"","street":"POPPY CLOSE","locality":"SELBY","town":"SELBY","district":"SELBY","county":"NORTH YORKSHIRE"}
{"price":"130000","date":14274,"postcode1":"YO8","postcode2":"9XP","type":"detached","is_new":0,"duration":"freehold","addr1":"10","addr2":"","street":"HEATHER CLOSE","locality":"","town":"SELBY","district":"SELBY","county":"NORTH YORKSHIRE"}
{"price":"150000","date":18180,"postcode1":"YO8","postcode2":"9XP","type":"detached","is_new":0,"duration":"freehold","addr1":"11","addr2":"","street":"HEATHER CLOSE","locality":"","town":"SELBY","district":"SELBY","county":"NORTH YORKSHIRE"}
{"price":"157000","date":18088,"postcode1":"YO8","postcode2":"9XP","type":"detached","is_new":0,"duration":"freehold","addr1":"12","addr2":"","street":"HEATHER CLOSE","locality":"","town":"SELBY","district":"SELBY","county":"NORTH YORKSHIRE"}
{"price":"134000","date":17333,"postcode1":"YO8","postcode2":"9XP","type":"semi-detached","is_new":0,"duration":"freehold","addr1":"16","addr2":"","street":"HEATHER CLOSE","locality":"","town":"SELBY","district":"SELBY","county":"NORTH YORKSHIRE"}
{"price":"250000","date":13405,"postcode1":"YO8","postcode2":"9YA","type":"detached","is_new":0,"duration":"freehold","addr1":"6","addr2":"","street":"YORKDALE COURT","locality":"HAMBLETON","town":"SELBY","district":"SELBY","county":"NORTH YORKSHIRE"}
{"price":"59500","date":11166,"postcode1":"YO8","postcode2":"9YB","type":"semi-detached","is_new":0,"duration":"freehold","addr1":"4","addr2":"","street":"YORKDALE DRIVE","locality":"HAMBLETON","town":"SELBY","district":"SELBY","county":"NORTH YORKSHIRE"}
{"price":"142500","date":17648,"postcode1":"YO8","postcode2":"9YB","type":"semi-detached","is_new":0,"duration":"freehold","addr1":"4A","addr2":"","street":"YORKDALE DRIVE","locality":"HAMBLETON","town":"SELBY","district":"SELBY","county":"NORTH YORKSHIRE"}
{"price":"230000","date":15125,"postcode1":"YO8","postcode2":"9YD","type":"detached","is_new":0,"duration":"freehold","addr1":"1","addr2":"","street":"ONE ACRE GARTH","locality":"HAMBLETON","town":"SELBY","district":"SELBY","county":"NORTH YORKSHIRE"}
{"price":"250000","date":15950,"postcode1":"YO8","postcode2":"9YD","type":"detached","is_new":0,"duration":"freehold","addr1":"3","addr2":"","street":"ONE ACRE GARTH","locality":"HAMBLETON","town":"SELBY","district":"SELBY","county":"NORTH YORKSHIRE"}

将 CSV 转换为 Parquet

反过来也一样——我们可以轻松读取新的 CSV 文件，并将其输出为 Parquet 文件。本地文件 house_prices.csv 可以在 ClickHouse 中使用 file 表函数读取，而 ClickHouse 会根据文件名以 .parquet 结尾这一点，将文件输出为 Parquet 格式 (或者我们也可以添加 FORMAT Parquet 子句) ：

./clickhouse local -q "SELECT *
FROM file('house_prices.csv')
INTO OUTFILE 'house_prices.parquet'"

正如上文所述，你可以结合 clickhouse local 使用任意一种 ClickHouse 输入和输出格式，轻松将文件转换为不同格式。