Table: population_history
Columns: year
1851
1861
1871
1881

Question: What's the 1851 when 1881 had 764?
SQL: SELECT "1851" FROM population_history WHERE "1881" = '764'

Question: What's the year when 1851 had 634?
SQL: SELECT "year" FROM population_history WHERE "1851" = '634'

Question: What's the 1881 when 1871 had 1017?
SQL: SELECT "1881" FROM population_history WHERE "1871" = '1017'

Question: What's the 1861 had 1991 in 1881 and a year of year?
SQL: SELECT "1861" FROM population_history WHERE "year" = 'year' AND "1881" = '1991'

Question: What's the 1881 with 1961 in 1851?
SQL: SELECT "1881" FROM population_history WHERE "1851" = '1961'