MySQL federated query

The joint query

• The cartesian product
• • Multiple tables query one data
  • join Implement multi table query
  • join Query multiple tables
• Inside and outside connection
• • Internal connection
  • External connection
• Self join
• Subquery
• • Use caution
• Merge query

The cartesian product

The cartesian product ： The core operation of multi table query . The calculation of Cartesian product is very simple , It's similar to permutation and combination . Cartesian product is an operation between any two tables .
give an example ：


Cartesian product calculation process ： First take the first record of the first form , And every record in the second table ÿ