Mybatis nine dynamic tags

1. If: You can judge, I can judge too!

<select id="count" resultType="">
	select count(*) from user where <if test="id != null">id = #{id}</if> and username = 'xiaoming'
</select>

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• If the incoming id is not null, then SQL will be spliced ​​id = #{id}
• If the incoming id is null, the final SQL statement becomes select count(*) from user where and username = ‘xiaoming’. There will be problems with this statement, and then the where tag should be grandly launched

2. Where: With me, SQL statement splicing conditions are all about the clouds!

<select id="count" resultType="">
	select count(*) from user
	<where>
		<if test="id != null">id = #{id}</if>
		and username = 'xiaoming'
    </where>
 </select>

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• The where element will only insert the WHERE clause if the condition of at least one child element returns the SQL clause.
• If the beginning of the statement is AND or OR, the where element will also remove them. You can also customize this processing rules through the trim tag

3. Trim: I will make the decision on my territory!

Trim tags are generally used to splice and remove prefixes and suffixes of SQL.

Properties in the trim tag

<select id="count" result="">
	select count(*) from user
	<trim prefix ="where" prefixOverrides="and | or">
		<if test="id != null">id = #{id}</if>
		<if test="username != null"> and username = #{username}</if>
	</trim>
</select>

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AI writing code

• If there is one of id or username that is not empty, add where before the statement. If where is followed by and or or will be automatically removed
• If both id or username are empty, nothing is spliced

4. set: Believe me, no mistakes!

<update id="UPDATE" parameterType="User">
	update user
    <set>
    	<if test="name != null">name = #{name},</if> 
        <if test="password != null">password = #{password},</if> 
        <if test="age != null">age = #{age},</if> 
   	</set>
</update>

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• At least one of the three ifs is not empty. Set will be added in front to automatically remove excess commas at the tail

5. foreach: You have for, I have foreach

Properties in the foreach tag

collection：

• If the parameter type is List, the value is list

<select id="count" resultType="">
	select count(*) from user where id in
  	<foreach collection="list" item="item" index="index" open="(" separator="," close=")">
        #{item}
  	</foreach>
</select>

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• If the parameter type is an array, the value is array

<select id="count" resultType="">
	select * from user where id inarray
  	<foreach collection="array" item="item" index="index" open="(" separator="," close=")">
        #{item}
  	</foreach>
</select>

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• If the parameter type is Map, the key of the parameter type is Map

6. Choose: I chose you, you chose me!

<select id="count" resultType="Blog">
	select count(*) from user
  	<choose>
    	<when test="id != null">
      		and id = #{id}
    	</when>
    	<when test="username != null">
      		and username = #{username}
    	</when>
    	<otherwise>
      		and age = 18
    	</otherwise>
  	</choose>
</select>

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• When neither id nor username is empty, then choose two (the former is preferred)
• If all are empty, then choose the
• If there is only one of id and username that is not empty, then choose the one that is not empty

7. SQL: It is equivalent to increasing the code in Java, and it needs to be used in conjunction with include.

<sql id="table"> user </sql>

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8. include: equivalent to method calls in Java

<select id="count" resultType="">
	select count(*) from <include refid="table (id value in sql tag)"/>
</select>

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9. bind: reprocessing the data

<select id="count" resultType="">
	select count(*) from user
	<where>
		<if test="name != null">
			<bind name="name" value="'%' + username + '%'"
			name = #{name}
		</if>
</select>

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