How to query the number of records when there is the DISTINCT keyword in the SQL multi-table join query string?

— table1 has 1 record

select * from t_test01

tid    value   
1      1.5

— table2 has 4 record

select * from t_test02

tid    value   
1      1.5
2      1.54
3      1.5
4      1.5

— t_test01.value = t_test02.value, 3 record

select t_test01.tid, t_test02.value
from 
t_test01, t_test02
where 
t_test01.value = t_test02.value

tid    value
1      1.5
1      1.5
1      1.5

— DISTINCT t_test01.value = t_test02.value, 1 record

select DISTINCT t_test01.tid, t_test02.value
from 
t_test01, t_test02
where 
t_test01.value = t_test02.value

tid    value
1      1.5

How to query the number of records when there is the DISTINCT keyword in the SQL multi-table join query string?

select COUNT(DISTINCT t_test01.tid, t_test02.value) ??
from 
t_test01, t_test02
where 
t_test01.value = t_test02.value

Answer

A quick-and-dirty approach is to use strings:

select count(distinct concat(t_test01.tid, '|', t_test02.value))
from t_test01 join
     t_test02 
     on t_test01.value = t_test02.value;

Otherwise, you can use a subquery or CTE:

select count(*)
from (select distinct t_test01.tid, t_test02.value
      from t_test01 join
           t_test02 
           on t_test01.value = t_test02.value
     ) t;

Note the use of proper, explicit, standard, readable JOIN syntax in both cases. Never use commas in the FROM clause.