Table schema:
CREATE TABLE singer_in_concert (concert_id VARCHAR); CREATE TABLE concert (concert_name VARCHAR, theme VARCHAR, concert_id VARCHAR)

Question: Show the name and theme for all concerts and the number of singers in each concert.
SQL: SELECT T2.concert_name, T2.theme, COUNT(*) FROM singer_in_concert AS T1 JOIN concert AS T2 ON T1.concert_id = T2.concert_id GROUP BY T2.concert_id