Semi joins
Semi joins are the opposite of anti joins: an anti-anti join, if you like.
A semi join returns the rows of the first table where it can find a match in the second table. The principle is shown in this diagram.
The syntax is the same as for other join types; simply swap the other join function for semi_join()
semi_join(a_tibble, another_tibble, by = c("id_col1", "id_col2"))
You may have spotted that the results of a semi join plus the results of an anti join give the orignial table. So, regardless of the table contents or how you join them, semi_join(A, B) plus anti_join(A, B) will return A (though maybe with the rows in a different order).
Bài tập này là một phần của khóa học
Introduction to Spark with sparklyr in R
Hướng dẫn bài tập
A Spark connection has been created for you as spark_conn. Tibbles attached to the track metadata and artist terms stored in Spark have been pre-defined as track_metadata_tbl and artist_terms_tbl respectively.
- Use a semi join to join the artist terms to the track metadata by the
artist_idcolumn. Assign the result tojoined. - Use
sdf_dim()to determine how many rows and columns there are in the joined table.
Bài tập tương tác thực hành trực tiếp
Hãy thử làm bài tập này bằng cách hoàn thành đoạn mã mẫu này.
# track_metadata_tbl and artist_terms_tbl have been pre-defined
track_metadata_tbl
artist_terms_tbl
# Semi join artist terms to track metadata by artist_id
joined <- ___
# How many rows and columns are in the joined table?
___