Essential conditional query statements for MySQL database

1. Basic grammar

SELECT
	Query list FROM
	Table name WHERE
	filter expression;

2. Filter by conditional expression

Conditional operators: >,<,=,!=,<>,>=,<=
The conditional operator cannot evaluate to null .

#Filter all information of employees whose salary is greater than 12000 in the employees table SELECT
	*
FROM
	employees
WHERE
	salary>12000;

3. Filter by logical expression

Logical operators: &&,||,!,and,or,not
Used to connect conditional expressions

SELECT
	*
FROM
	employees
WHERE
	salary>12000 AND salary<16000;

4. Fuzzy query

Keywords: like,between and,in,is null,is not null

①like: Usually used with wildcards. Wildcards: % represents any number of characters, _ represents an arbitrary character. If the wildcard itself needs to be used, use the \ escape character, such as \_ .

#Query all information of employees whose second letter of name is aSELECT
	*
FROM
	employees
WHERE
	last_name LIKE '_a%';

②between and: contains two critical values. Note that the order of the two critical values ​​cannot be reversed.

SELECT
	*
FROM
	employees
WHERE
	salary BETWEEN 10000 AND 16000;

③in: Determine whether the value of a field belongs to an item in the in list. Wildcards are not supported.

#Query all information of employees with job numbers SA_MAN and PR_REPSELECT
	*
FROM
	employees
WHERE
	job_id IN('SA_MAN','PR_REP');

④is null (is not null): is empty and not empty

#Query all information of employees without bonus SELECT
	*
FROM
	employees
WHERE
	commission_pct IS NULL;

5. Safety is equal to

Symbol: <=> means equal, which can replace is , =

SELECT
	*
FROM
	employees
WHERE
	commission_pct <=> NULL;

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