一个查询中有多个表计数

Question

I've done some searching around but I haven't found a clear answer and explanation to my question.

I have 5 tables called table1, table2, table3, table4 and table5 and I want to do COUNT(*) on each of the tables to get the number of rows.

Should I try to combine these into one query or use 5 separate queries? I have always been taught that the least number of queries the better so I am guessing I should try to combine them into one query.

One way of doing it is to use UNION but does anyone know what the most efficient way of doing this is and why?

Thanks for any help.

Answer 1

Assuming you just want a count(*) from each one, then

SELECT
   ( SELECT count(*) from table1 ) AS table1,
   ( SELECT count(*) from table2 ) AS table2,
   ( SELECT count(*) from table3 ) AS table3,
   etc...
)

would give you those counts as a single row. The DB server would still be running n+1 queries (n tables, 1 parent query) to get those counts, but it'd be the same story if you were using a UNION anyways. The union would produce multiple rows with 1 value in each, vs the 1 row with multiple values of the subselect method.

Answer 2

Provided you have read access to this (so rather not on shared hosting where you don't have your actual own database instance) you could read that info from the INFORMATION_SCHEMA TABLES table, that does have a TABLE_ROWS column.

(Be aware of what it says for InnoDB tables there – so if you don't you MyISAM and need the precise counts, the other answer has the better solution.)