Below is the list of tables from the database along with their columns and datatype. Need to generate an accurate SQL query for the question.
Only use the columns present in the respective table. Only use the columns which are required to generate the SQL query. Only use table joins if required and not otherwise.

### Tables ###
TABLE: CREATE TABLE table_45645 ( "Position" real, "best year" text, "best 2-year period" text, "best 3-year period" text, "best 5-year period" text, "best 10-year period" text, "best 15-year period" text )

### Question ###
Which best 5-year period has a best 15-year period of alekhine; lasker?

### Accurate SQL ###

SELECT "best 5-year period" FROM table_45645 WHERE "best 15-year period" = 'alekhine; lasker'