mybatis study notes four

Recorded dynamic sql common tags:

1.where

Conditions are generally used as a data addition operation in

example:

 <select id="selectByRoleId" resultMap="resource">
        select * from resource <where> role_id = #{roleId} </where>

2.if

Generally used as a query, modify, or delete some of the data splicing conditions.

Analyzing the condition test field

example:

<select id="findCountByContditon" resultType="int" parameterType="com.example.demo.po.User">
        select coalesce(count(id),0) from user <where> <if test="name != null and name != ''"> name like #{name} </if> <if test="createTime != null"> and create_time &lt; #{createTime} </if> </where> </select>

The above sql there is a problem, that name if conditions are not met, creatTime condition is satisfied, then the sql error. In general, when determining if there are multiple, where 1 = 1 is added before if under conditions where.

<select id="findCountByContditon" resultType="int" parameterType="com.example.demo.po.User">
        select coalesce(count(id),0) from user <where> 1=1 <if test="name != null and name != ''"> and name like #{name} </if> <if test="createTime != null"> and create_time &lt; #{createTime} </if> </where> </select>

3.foreach

Generally used to add bulk data

field generally collection list, array, map type, the value of the parameters passed

separator delimiter field value, i.e. what divides

alias for each item field is the loop element

each subscript index field value at the first cycle

open field value prefix value

close suffix value field value

example:

<insert id="saveAll" parameterType="java.util.List">
        insert into user(name,role_id,password) values <foreach collection="users" separator="," item="user" index="index" > ( #{user.name}, #{user.roleId}, #{user.password}) </foreach> </insert>

4.set

Generally used in the modified data, if often appear together with

<update id="updateByCondition">
        update user
        <set> <if test="name != null and name != ''"> name = #{name}, </if> <if test="password != null and password != ''"> password = #{password}, </if> <if test="createTime != null"> create_time = @{createTime} </if> </set> where id = #{id} </update>

5.include

Generally it is used as the constant field extraction

<sql id="user_columns">
        id, name,role_id,password,create_time,update_time
    </sql>

<select id="selectById" resultMap="user">
        select <include refid="user_columns"/> from user where id = #{id} </select>

6.trim

prefix value field prefix is ​​added

suffix added suffix value field

prefixOverrides field delete prefix value

suffixOverrides field delete suffix value

<update id="updateByCondition">
        update user  set
        <trim suffixOverrides=","> <if test="name != null and name != ''"> name = #{name}, </if> <if test="password != null and password != ''"> password = #{password}, </if> <if test="createTime != null"> create_time = @{createTime}, </if> </trim> where id = #{id} </update>

If the password is empty, the other is not empty. sql becomes

update user set name = #{name}, create_time = #{createTime} where id = #{id }

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