Zero foundation self-study SQL course | union joint query

Hello everyone , I'm Ning Yi .

Today's talk SQL Course No 13 course ：UNION The joint query .

In the first two classes JOIN Connect , Horizontal column consolidation can be performed on multiple tables .

This lesson is about UNION The operator , Is to combine multiple query results , Merge vertically by row .

Basic grammar ：

SELECT  < Field name >  FROM  < Table name >
UNION
SELECT  < Field name >  FROM  < Table name >

1、 Single table joint query

UNION When merging result sets , If there are duplicate rows in the merged result set , Only one line will be retained .

example ： Query out Students In the table , Number students Sid by 2 To 5 And 1 To 3 Your record passed UNION unite .

Instance analysis ： UNION The result set above has Sid by 1、2、3 The record of ,UNION The following result sets have Sid by 2、3、4、5 The record of .UNION United time , Only duplicate lines will be retained （ namely Sid by 2、3 The record of ） One of the lines .

SELECT * FROM Students
WHERE Sid BETWEEN 2 AND 5
UNION
SELECT * FROM Students
WHERE Sid BETWEEN 1 AND 3

If you want to keep all duplicate lines , have access to UNION All Merge result set .

SELECT * FROM Students
WHERE Sid BETWEEN 2 AND 5
UNION ALL
SELECT * FROM Students
WHERE Sid BETWEEN 1 AND 3

2、 Joint query of different tables

UNION The number of columns in the upper and lower result sets of the operator must be equal , Otherwise, an error will be reported .

Column name of the final consolidated table , Determined by the result set above .

example ： take Students In the table , Birthday Sage stay 1995 Names of students born before Sname And Teachers Name of teacher in the table Tname Merge .

SELECT Sname FROM Students
WHERE Sage < '1995-01-01'
UNION
SELECT Tname FROM Teachers

3、 Statement execution order

If it's followed by ORDER BY、LIMIT Clause , These clauses do not apply only to the last query statement , But will wait UNION Merge the result sets before executing .

SELECT * FROM Students
WHERE Sid BETWEEN 2 AND 5
UNION
SELECT * FROM Students
WHERE Sid BETWEEN 1 AND 3
ORDER BY Sid

ORDER BY After the result set is merged , And then sort the whole result set , Not just for the last query statement .

LIMIT Clause will also wait for the result set to be merged , Take the previous records .

SELECT * FROM Students
WHERE Sid BETWEEN 2 AND 5
UNION
SELECT * FROM Students
WHERE Sid BETWEEN 1 AND 3
LIMIT 2

Next class , Let's talk about aggregate functions .

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