golang conditional query

Golang is a fast and efficient programming language. Now more and more programmers are turning to it because its learning curve is flatter than other languages ​​and its concurrency performance is very outstanding, which makes it popular in web development. It has a wide range of applications in the field of back-end development.

When developing web applications, it is inevitable to perform database query operations, and conditional query is a very important part of it. This article will introduce how to use Golang to perform conditional query.

Step 1: Connect to the database

Before performing conditional query, you need to connect to the database first. Golang has many open source database drivers to choose from, such as Gorm, sqlx and go-sql-driver. This article uses Gorm as an example to demonstrate how to perform conditional queries.

Using Gorm to connect to the database requires the following steps:

1. Install Gorm

You can use the command line to install:

go get -u github.com/jinzhu/gorm

2. Import the Gorm package

After importing the Gorm package, you need to import the database driver package, for example, import the Mysql driver package:

import (

  "github.com/jinzhu/gorm"
  _ "github.com/jinzhu/gorm/dialects/mysql"

)

3. Connect to the database

When connecting to the database, you need to provide the database connection information, for example:

db, err := gorm.Open("mysql", "user:password@tcp(dbhost:port)/dbname?charset=utf8&parseTime=True&loc=Local")

This statement will create a DB The object is connected to the database, which includes information such as user name, password, database host and port number, as well as information such as character set, time format and time zone.

Step 2: Build query conditions

After connecting to the database, you need to build query conditions. The query condition is a key part, it specifies what data we want to get from the database. Gorm uses chain methods to build query conditions, and it provides some commonly used methods, such as Where(), Not(), Or(), and And(), etc.

1. Where() method

The Where() method is used to filter records that meet specified conditions, for example:

db.Where("age > ; ?", 18)

This statement will return records whose age is greater than 18.

2. Not() method

The Not() method is used to filter records that do not meet the specified conditions, for example:

db.Not("age > ?", 18)

This statement will return records whose age is less than or equal to 18.

3. Or() method and And() method

Or() method and And() method are used to construct complex query conditions, for example:

db.Where("age > ?", 18).Or("weight > ?", 60)

This statement will return records whose age is greater than 18 or whose weight is greater than 60.

db.Where("age > ?", 18).And("name = ?", "Tom")

This statement will return people whose age is greater than 18 and whose name is Tom record of.

In addition to the above methods, Gorm also provides some other methods, such as Find(), First() and Last(), etc., which can be used to obtain data from the database.

Step 3: Execute the query

After constructing the query conditions, you need to execute the query statement to obtain data from the database. Gorm's query methods are also very diverse. It provides methods such as Find(), First(), Last(), and Count() to ensure that the query results meet expectations.

1. Find() method

The Find() method can obtain all records that meet the conditions from the database, for example:

var users []User
db.Where("age > ?", 18).Find(&users)

This statement will return all user information whose age is greater than 18 and store it in the users variable.

2. First() method and Last() method

The First() method and Last() method can obtain the first record and the last record that meet the conditions from the database A record, for example:

var user User
db.Where("age > ?", 18).First(&user)

This statement will return those whose age is greater than 18 The first user information and stores it in the user variable.

var user User
db.Last(&user)

This statement will return the last user's information and store it in the user variable.

3. Count() method

The Count() method is used to query the number of records that meet specified conditions, for example:

var count int
db.Table("users").Where("age > ?", 18).Count(&count)

This statement will return the number of users whose age is greater than 18 and store it in in the count variable.

Summary

Conditional query is one of the basic operations on the database, which allows us to obtain the data we really need from the database. The Gorm library of Golang language provides a very rich and flexible query method, allowing us to easily construct query conditions that meet our needs. It also provides a variety of query operations (such as Find(), First(), Last() and Count(), etc.) to ensure that the query results are as expected.

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