SQL modification statement

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If we want to modify the data of tables in the database , At this time, we will use UPDATE sentence .

UPDATE The basic syntax of a sentence is ：

UPDATE < Table name > SET Field 1= value 1, Field 2= value 2, … WHERE …;

for example , We want to update employees surface id=100 The record of last_name and salary These two fields , Write first UPDATE employees SET name=” eldest brother ”, salary = 54000, And then in WHERE Clause to write the filter conditions of the rows that need to be updated id=100：

namely UPDATE employees SET last_name=‘ eldest brother ’, salary=54000 WHERE employee_id=100;

– Before the change ：

– After modification ：

Here we find UPDATE Of the statement WHERE Conditions and SELECT Of the statement WHERE The conditions are actually the same , Therefore, multiple records can be updated at one time ：

UPDATE employees SET last_name=‘TEST’, salary=54000 WHERE employee_id>=101 and employee_id<=105;

– Before the change ：

– After modification ：

stay UPDATE In the sentence , We can also use expressions when updating fields .

for example , Put all the 50000 For employees with a salary of more than yuan, add 666 element ：

UPDATE employees SET salary=54000+666 WHERE salary >= 50000;

– Query results

among ,SET salary=54000+666 It's for the current line salary The value of the field is added 666.

If WHERE The condition does not match any record ,UPDATE Statements do not report errors , But no records will be updated .

UPDATE Statement can have no WHERE Conditions , for example ：

UPDATE employees SET salary=54000+666

This is the time , All records of the entire table will be updated . therefore , In execution UPDATE Be very careful when making statements , It's better to use SELECT Statement to test WHERE Whether the condition filters out the desired recordset , And then use UPDATE updated .

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