Introduction to the use of conditional query statements in Mysql database

This article brings you an introduction to the use of conditional query statements in Mysql database. It has certain reference value. Friends in need can refer to it. I hope it will be helpful to you. .

For analysts, the most commonly used query statement in Mysql database is the select query statement. This article mainly introduces the query statement of Mysql database.

1. Single table query

1. Query with conditions
Basic syntax: select * from table name where condition;
1) Range query:
eg: where field between 10 and 100;
2) Fuzzy query
eg: where field like'%uncertain%'
Remarks: % represents multiple characters, _underscore represents one character, ^a Represents data starting with a, a$ represents data ending with a, [abc] matches any character contained. (Related recommendations: MySQL Tutorial)

2. Multi-condition query
Note: When and and or are used at the same time, and has a higher priority than or. If you want to give priority to conditions with or, just put parentheses on the partial conditions with or.

3. Sorting
Basic syntax: order by field.
Remarks:The default is ascending order, ASC ascending order, DESC descending order
Limit the number of records: limit 100

4. Aggregation function
count(): count the number of records
avg(): Average
max(): Maximum value
min(): Minimum value
sum(): Sum

5. Grouping
Basic syntax: select field 1, field 2, aggregate function from table name group by field 1, field 2
Remarks: group by and having are used together, mainly to filter the grouping results

2. Multi-table association query
1. Inner join-->inner join

Inner join is an equivalent join, which obtains records of field matching relationships in two tables. It can be omitted and written as join,
It can be understood as the "intersection" in the collection concept, and the associated fields exist with the records of the two tables at the same time.

2. Left join-->left join

Left join obtains all records of the main table on the left, even if there is no corresponding data in the right table.

3. Right join-->right join

Right join gets all the records of the main table on the right, even if there is no corresponding data in the left table.

4. Connector -->union
union is used to connect the results of more than two select statements, combine the results into a result set, and delete duplicate data
Basic Syntax: select field from table 1 union [all|distinct] select field from table 2
Remarks: union means union distinct; if it is union all, it returns a result set with duplicate data; when using union, the selected The displayed content will be named after the field names of the previous table.

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