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454 lines
9.9 KiB
Markdown
454 lines
9.9 KiB
Markdown
# 灵活的 QueryWrapper
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在 [增删改](./add-delete-update) 和 [查询和分页](./query) 章节中，我们随时能看到 QueryWrapper 的身影，QueryWrapper 是用于构造 Sql 的
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强有力工具，也是 Mybatis-Flex 的亮点和特色。
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::: tip 提示
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QueryWrapper 可以被序列化通过 RPC 进行传输，因此，在微服务项目中，我们可以在客户端（网关、Controller 层等）构造出 QueryWrapper，传给
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Provider 层进行查询返回数据。
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:::
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## QueryWrapper 的使用
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以下代码是一个完整 Spring Controller 的示例：
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```java
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@RestController
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public class AccountController {
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@Autowired
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AccountMapper accountMapper;
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@GetMapping("/accounts")
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List<Account> selectList() {
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//构造 QueryWrapper，也支持使用 QueryWrapper.create() 构造，效果相同
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QueryWrapper query = new QueryWrapper();
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query.where(ACCOUNT.ID.ge(100));
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//通过 query 查询数据列表返回
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return accountMapper.selectListByQuery(query);
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}
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}
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```
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在以上的示例中，其核心代码如下所示：
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```java
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//构造 QueryWrapper
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QueryWrapper query = new QueryWrapper();
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query.where(ACCOUNT.ID.ge(100));
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//通过 query 查询数据列表
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accountMapper.selectListByQuery(query);
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```
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以上代码执行的 Sql 如下：
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```sql
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select * from tb_account
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where id >= 100
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```
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## select *
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```java
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QueryWrapper query = new QueryWrapper();
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query.select(ACCOUNT.ID, ACCOUNT.USER_NAME)
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.from(ACCOUNT)
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```
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其查询生成的 Sql 如下：
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```sql
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SELECT id, user_name FROM tb_account
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```
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## select ... as
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```java
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QueryWrapper query = new QueryWrapper()
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.select(
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ACCOUNT.ID.as("accountId")
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, ACCOUNT.USER_NAME
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.from(ACCOUNT.as("a"));
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```
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其查询生成的 Sql 如下：
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```sql
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SELECT a.id as accountId, a.user_name
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FROM tb_account AS a
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```
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## select 多张表
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```java
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QueryWrapper query = new QueryWrapper()
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.select(
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ACCOUNT.ID
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, ACCOUNT.USER_NAME
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, ARTICLE.ID.as("articleId")
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, ARTICLE.TITLE)
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.from(ACCOUNT.as("a"), ARTICLE.as("b"))
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.where(ACCOUNT.ID.eq(ARTICLE.ACCOUNT_ID));
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```
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其查询生成的 Sql 如下：
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```sql
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SELECT a.id, a.user_name, b.id AS articleId, b.title
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FROM tb_account AS a, tb_article AS b
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WHERE a.id = b.account_id
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```
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## select function
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```java
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QueryWrapper query=new QueryWrapper()
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.select(
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ACCOUNT.ID,
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ACCOUNT.USER_NAME,
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max(ACCOUNT.BIRTHDAY),
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avg(ACCOUNT.SEX).as("sex_avg")
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).from(ACCOUNT);
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```
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其查询生成的 Sql 如下：
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```sql
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SELECT id, user_name, MAX(birthday), AVG(sex) AS sex_avg
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FROM tb_account
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```
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## where
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```java
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QueryWrapper queryWrapper=QueryWrapper.create()
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.select()
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.from(ACCOUNT)
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.where(ACCOUNT.ID.ge(100))
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.and(ACCOUNT.USER_NAME.like("michael"));
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```
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其查询生成的 Sql 如下：
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```sql
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SELECT * FROM tb_account
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WHERE id >= ?
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AND user_name LIKE ?
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```
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## where 动态条件 1
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```java 1,4
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boolean flag = false;
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QueryWrapper queryWrapper = QueryWrapper.create()
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.select().from(ACCOUNT)
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.where(flag ? ACCOUNT.ID.ge(100) : noCondition())
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.and(ACCOUNT.USER_NAME.like("michael"));
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```
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其查询生成的 Sql 如下：
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```sql
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SELECT * FROM tb_account
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WHERE user_name LIKE ?
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```
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## where 动态条件 2
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```java 1,4
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boolean flag = false;
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QueryWrapper queryWrapper = QueryWrapper.create()
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.select().from(ACCOUNT)
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.where(ACCOUNT.ID.ge(100).when(flag)) // when....
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.and(ACCOUNT.USER_NAME.like("michael"));
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```
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其查询生成的 Sql 如下：
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```sql
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SELECT * FROM tb_account
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WHERE user_name LIKE ?
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```
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## where select
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```java
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QueryWrapper queryWrapper = QueryWrapper.create()
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.select()
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.from(ACCOUNT)
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.where(ACCOUNT.ID.ge(
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select(ARTICLE.ACCOUNT_ID).from(ARTICLE).where(ARTICLE.ID.ge(100))
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));
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```
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其查询生成的 Sql 如下：
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```sql
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SELECT * FROM tb_account
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WHERE id >=
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(SELECT account_id FROM tb_article WHERE id >= ? )
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```
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## where exists, not exists
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```java
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QueryWrapper queryWrapper=QueryWrapper.create()
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.select()
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.from(ACCOUNT)
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.where(ACCOUNT.ID.ge(100))
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.and(
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exists( // or notExists(...)
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selectOne().from(ARTICLE).where(ARTICLE.ID.ge(100))
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)
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);
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```
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其查询生成的 Sql 如下：
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```sql
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SELECT * FROM tb_account
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WHERE id >= ?
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AND EXISTS (
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SELECT 1 FROM tb_article WHERE id >= ?
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)
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```
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## and (...) or (...)
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```java
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QueryWrapper queryWrapper=QueryWrapper.create()
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.select()
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.from(ACCOUNT)
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.where(ACCOUNT.ID.ge(100))
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.and(ACCOUNT.SEX.eq(1).or(ACCOUNT.SEX.eq(2)))
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.or(ACCOUNT.AGE.in(18,19,20).and(ACCOUNT.USER_NAME.like("michael")));
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```
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其查询生成的 Sql 如下：
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```sql
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SELECT * FROM tb_account
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WHERE id >= ?
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AND (sex = ? OR sex = ? )
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OR (age IN (?,?,?) AND user_name LIKE ? )
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```
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## group by
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```java
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QueryWrapper queryWrapper=QueryWrapper.create()
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.select()
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.from(ACCOUNT)
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.groupBy(ACCOUNT.USER_NAME);
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```
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其查询生成的 Sql 如下：
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```sql
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SELECT * FROM tb_account
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GROUP BY user_name
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```
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## having
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```java
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QueryWrapper queryWrapper=QueryWrapper.create()
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.select()
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.from(ACCOUNT)
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.groupBy(ACCOUNT.USER_NAME)
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.having(ACCOUNT.AGE.between(18,25));
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```
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其查询生成的 Sql 如下：
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```sql
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SELECT * FROM tb_account
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GROUP BY user_name
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HAVING age BETWEEN ? AND ?
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```
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## orderBy
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```java
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QueryWrapper queryWrapper=QueryWrapper.create()
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.select()
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.from(ACCOUNT)
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.orderBy(ACCOUNT.AGE.asc(), ACCOUNT.USER_NAME.desc().nullsLast());
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```
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其查询生成的 Sql 如下：
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```sql
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SELECT * FROM tb_account
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ORDER BY age ASC, user_name DESC NULLS LAST
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```
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## join（left join，inner join...）
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```java
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QueryWrapper queryWrapper=QueryWrapper.create()
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.select()
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.from(ACCOUNT)
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.leftJoin(ARTICLE).on(ACCOUNT.ID.eq(ARTICLE.ACCOUNT_ID))
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.innerJoin(ARTICLE).on(ACCOUNT.ID.eq(ARTICLE.ACCOUNT_ID))
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.where(ACCOUNT.AGE.ge(10));
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```
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其查询生成的 Sql 如下：
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```sql
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SELECT * FROM tb_account
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LEFT JOIN tb_article ON tb_account.id = tb_article.account_id
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INNER JOIN tb_article ON tb_account.id = tb_article.account_id
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WHERE tb_account.age >= ?
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```
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## join on 多个条件
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```java
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QueryWrapper queryWrapper = QueryWrapper.create()
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.select()
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.from(ACCOUNT)
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.leftJoin(ARTICLE).on(
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ACCOUNT.ID.eq(ARTICLE.ACCOUNT_ID).and(ACCOUNT.AGE.eq(18))
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)
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.where(ACCOUNT.AGE.ge(10));
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```
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其查询生成的 Sql 如下：
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```sql
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SELECT * FROM tb_account LEFT JOIN tb_article
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ON tb_account.id = tb_article.account_id AND tb_account.age = ?
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WHERE tb_account.age >= ?
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```
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## join select
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```java
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QueryWrapper queryWrapper = QueryWrapper.create()
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.select()
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.from(ACCOUNT)
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.leftJoin(
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select().from(ARTICLE).where(ARTICLE.ID.ge(100))
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).as("a").on(
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ACCOUNT.ID.eq(raw("a.id"))
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)
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.where(ACCOUNT.AGE.ge(10));
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```
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其查询生成的 Sql 如下：
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```sql
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SELECT * FROM tb_account
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LEFT JOIN (SELECT * FROM tb_article WHERE id >= ? ) AS a
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ON tb_account.id = a.id
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WHERE tb_account.age >= ?
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```
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## union, union all
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```java
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QueryWrapper query = new QueryWrapper()
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.select(ACCOUNT.ID)
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.from(ACCOUNT)
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.orderBy(ACCOUNT.ID.desc())
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.union(select(ARTICLE.ID).from(ARTICLE))
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.unionAll(select(ARTICLE.ID).from(ARTICLE));
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```
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其查询生成的 Sql 如下：
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```sql
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(SELECT id FROM tb_account ORDER BY id DESC)
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UNION (SELECT id FROM tb_article)
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UNION ALL (SELECT id FROM tb_article)
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```
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## limit... offset
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::: tip 提示
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在 "limit... offset" 的示例中，Mybatis-Flex 能够自动识别当前数据库👍，并根据数据库的类型生成不同的 SQL，用户也可以很轻易的通过 DialectFactory 注册（新增或改写）自己的实现方言。
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:::
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```java
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QueryWrapper queryWrapper = QueryWrapper.create()
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.select()
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.from(ACCOUNT)
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.orderBy(ACCOUNT.ID.desc())
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.limit(10)
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.offset(20);
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```
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MySql 下执行的代码如下：
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```sql
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SELECT * FROM `tb_account` ORDER BY `id` DESC LIMIT 20, 10
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```
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PostgreSQL 下执行的代码如下：
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```sql
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SELECT * FROM "tb_account" ORDER BY "id" DESC LIMIT 20 OFFSET 10
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```
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Informix 下执行的代码如下：
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```sql
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SELECT SKIP 20 FIRST 10 * FROM "tb_account" ORDER BY "id" DESC
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```
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Oracle 下执行的代码如下：
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```sql
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SELECT * FROM (SELECT TEMP_DATAS.*,
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ROWNUM RN FROM (
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SELECT * FROM "tb_account" ORDER BY "id" DESC)
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TEMP_DATAS WHERE ROWNUM <=30)
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WHERE RN >20
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```
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Db2 下执行的代码如下：
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```sql
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SELECT * FROM "tb_account" ORDER BY "id" DESC
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OFFSET 20 ROWS FETCH NEXT 10 ROWS ONLY
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```
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Sybase 下执行的代码如下：
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```sql
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SELECT TOP 10 START AT 21 * FROM "tb_account" ORDER BY "id" DESC
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```
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Firebird 下执行的代码如下：
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```sql
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SELECT * FROM "tb_account" ORDER BY "id" DESC ROWS 20 TO 30
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```
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## 存在疑问？
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**疑问1：示例代码中的 QueryWrapper 所需要的 "ACCOUNT" 从哪里来的？**
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答：Mybatis-Flex 使用了 APT（Annotation Processing Tool）在项目编译的时候，
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会自动根据 Entity 类定义的字段生成 "ACCOUNT" 类以及 Entity 对应的 Mapper 类， 通过开发工具构建项目（如下图），
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或者执行 maven 编译命令: `mvn clean package` 都可以自动生成。这个原理和 lombok 一致。
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> 更多关于 APT 的配置，请进入 [APT 配置章节](../others/apt.md) 了解。
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## 特别注意事项!!!
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在 QueryWrapper 的条件构建中，如果传入 null 值，则自动忽略该条件，这有许多的好处，不需要额外的通过 `when()` 方法判断。但是也带来一些额外的知识记忆点，
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因此，正对这一点需要特别注意一下。
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例如：
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```java
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String userName = null;
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Integer id = null;
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QueryWrapper query1 = QueryWrapper.create()
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.where(ACCOUNT.AGE.ge(18))
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.and(ACCOUNT.USER_NAME.like(userName))
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.and(ACCOUNT.ID.ge(id));
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QueryWrapper query2 = QueryWrapper.create()
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.where(ACCOUNT.AGE.ge(18));
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```
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在以上的 `query1` 和 `query2` 中，它们构建出来的 SQL 条件是完全一致的，因为 Mybatis-Flex 会自动忽略 null 值的条件。 |