04·Merge, join, concat — combining tables

How does pd.concat([df1, df2]) differ from df1.merge(df2)?

• concat returns a Series, merge a DataFrame.
• They are synonyms.
• concat stacks rows (axis=0) or columns (axis=1) WITHOUT match logic — alignment by index or columns. merge joins on key values (like SQL JOIN) — can produce a cross product, requires compatible types.
• concat is deprecated since pandas 2.0.