问题
Is there a way to perform SQL queries on CSV text held in memory and read in from a Java Reader e.g. StringReader.
org.h2.tools.Csv.read(Reader reader, String[] colNames) would allow me to retrieve a result set containing all the rows and columns. However, I actually want to perform a query on the CSV text read from the Reader.
The background - I receive a file containing multiple CSV sections for each entity, see Can H2 Database query a CSV file containing multiple sections of different record groups?, and whilst parsing the file I store each of the CSV sections I need in Strings (a String for each one). This shouldn't bog down memory as I only keep the data in memory for a short time and each CSV section is relatively small). I need to perform queries on these CSV sections to build a document in a custom format.
I could write each CSV section to a file (as a set of files) and use CSVREAD, but I don't want to do that as I need my application to be as fast as possible and splitting and writing the sections to disk will thrash the hard drive to death.
回答1:
You could write a user defined function that returns a result set, and use that to generate the required rows. Within your user defined function, you can use the Csv tool from H2 (actually any Csv tool).
回答2:
This is not possible directly, since DBMS can usually only query their own optimized data storage. You have to import the text with the mentioned org.h2.tools.Csv.read into a table and perform the queries on that table. The table may be a temporary one, to prevent any writes on the disk, assuming the memory is sufficient.
来源：https://stackoverflow.com/questions/8115157/with-h2-database-can-i-perform-a-sql-query-on-csv-text-read-from-a-java-reader-e