# Summer self-study day 10 | database (III) – DQL

Time：2021-10-25

### DQL

#### Sort query

• `... order by keyword ASC/DESC`
• If ascending / descending order is not written, the default is ascending order

#### Aggregate function

• Take a column as a whole and do longitudinal calculation
• Null values are excluded
• Solution: use the ifnull() function or select a non empty column
• `count`Number of calculations
• `SELECT COUNT(english) FROM stu`or`SELECT COUNT(IFNULL(english, 0)) FROM stu`Null set to 0
• `max`, `min`, `sum`, `avg`The use method is the same as that of count

#### Grouping query

• `Group by group field`
• Query the data according to the grouping field. For example, if it is grouped according to gender, men and women are divided into two groups.
• The syntax of two qualifications:`where ...`or`having ...`
• difference:
• Where is limited before grouping and does not participate in grouping if the conditions are not met; Having is limited after grouping. If the grouping is not satisfied, it will not be queried
• Aggregate function cannot be used to judge where, but having can.

#### Paging query

• `Limit data start index, number of queries`
• `select * from stu limit 0, 3;`Start from 0 and check three records

#### Basic query

• `SELECT col1, col2, ... colN FROM t_table;`
• De duplication: use the distinct keyword
• `SELECT DISTINCT col1 FROM t_table;`
• Query the sum of two columns:
• `SELECT name, (math + english) FROM t_table`
• If a column of data is null, the operation result is null
• Alias a column: use the as keyword
• `SELECT name, (math + english) AS total FROM t_table`

#### Condition query

• where
• Operators >, <, =, > =,! =, < >, and/&&, or/||, not/|
• BETWEEN … AND
• `... BETWEEN 20 AND 30;`
• IN
• `... WHERE age IN (22, 18, 25);`
• Is / is not: judge whether it is null
• `... IS NULL`

#### Fuzzy query

• LIKE
• _ Underscore: single arbitrary character
• %Percent sign: multiple arbitrary characters
• Query data containing a character:
• `... where name like '% horse%';`//Query contains the name of the horse

This work adoptsCC agreement, reprint must indicate the author and the link to this article

## [hematemesis finishing] Super complete golang interview questions collection + golang Learning Guide + golang knowledge map + growth route

The brain map is constantly updated. Check the address onlineSubsequent articles and contents will be updated toGitHub projectWelcome to pay attention. Directory (Ctrl + F) Basic introduction Novice 50 mistakes that golang novices often make data type I don’t even know that nil slice is different from empty slice? Then the bat interviewer has to […]