MySQL Federated Query

Union query: Multiple queries (multiple SELECT statements), stitching on records (fields do not increase)

basic Syntax: Multiple SELECT statement composition: The number of fields that each SELECT statement obtains must be strictly consistent (but field type Independent)

Grammar

Select Statement 1Union [Union option]select Statement 2

Union option: As with the Select option, there are two

All: Keep all (regardless of duplicates)

DISTINCT: deduplication (entire repetition): default

For example

--Table A SELECT * from Prospect;+---------+-------+------------------------+|FName|LName|Addr|+---------+-------+------------------------+|Peter|Jones| 482Rush St., Apt.402 ||Bernice|Smith| 916Maple Dr.|+---------+-------+------------------------+--Table Two SELECT *  fromcustomer;+-----------+------------+---------------------+|Last_Name|First_Name|Address|+-----------+------------+---------------------+|Peterson|Grace| 16055Seminole Ave.||Smith|Bernice| 916Maple Dr.||Brown|Walter| 86021st St.|+-----------+------------+---------------------+--Table III SELECT *  fromVendor;+-------------------+---------------------+|Company|Street|+-------------------+---------------------+|Reddyparts, Inc.|  -Industrial Blvd.||Parts- to-Go, Ltd.|213B Commerce Park.|+-------------------+---------------------+--If all three tables have different column names, the Query methodSELECTfname, lname, addr fromProspectUNIONSELECTFirst_Name, last_name, address fromCustomerUNIONSELECTCompany"', street fromVendor;+-------------------+----------+------------------------+|FName|LName|Addr|+-------------------+----------+------------------------+|Peter|Jones| 482Rush St., Apt.402 ||Bernice|Smith| 916Maple Dr.||Grace|Peterson| 16055Seminole Ave.||Walter|Brown| 86021st St.||Reddyparts, Inc.|                |  -Industrial Blvd.||Parts- to-Go, Ltd.|                |213B Commerce Park.|+-------------------+----------+------------------------+--If you want to select all records, including duplicates, please click the First union keyword SELECTfname, lname, addr fromProspect - UNION  All - SELECTFirst_Name, last_name, address fromCustomer - UNION - SELECTCompany"', street fromVendor;

The meaning of a federated query:

1. Query the same table, but the requirements are different: such as inquiring students information, boys height ascending, girls height descending.

2. Multi-table query: The structure of multiple tables is exactly the same, and the stored data (structure) is the same.

Order by uses

In a federated query: the order by cannot be used directly, the query statement needs to use parentheses;

Also, order by takes effect: must be paired with Limit:limit to use the specified maximum number

.

-- the men are ordered by grade, and the women are sorted by flashbacks . (Select*fromwhere gender=orderby99999 )Union(Select*fromwhere gender = Order  by 99999)

MySQL Federated Query