04·Merge, join, concat — combining tables

What does a "left semi join" mean in pandas and how do you perform it?

• A semi join is the same as a left join.
• merge(how="semi") is a standard option.
• Rows from the left DataFrame that HAVE a match on the right — without adding right-side columns. Pandas has no how="semi" — pattern: df1[df1["k"].isin(df2["k"])] or df1.merge(df2[["k"]].drop_duplicates(), how="inner").
• Only SQL libraries support semi join.