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if label

if label test屬性中有一個，test屬性值是匹配OGNL必選判斷表達式，表達式的結果可以使true或者false，另外，所有非0的值都為true

1.數值型

(1)for example ：

If there is no special requirement when the parameter is numeric, you only need to judge whether it is null that will do .
<if test="id != null"></if>

(2)for example ：

If there is a special demand , For example, judge whether it is greater than a certain number . Just add the corresponding conditional judgment .
<if test='id != null and id > 28'></if>

(3)for example ：

mybatis There is another form for this greater than less than and so on .
<if test='id != null and id gt 28'></if>
<if test='id != null and id > 28'></if>  The two are the same 
<if test='id != null and id gte 28'></if>
<if test='id != null and id >= 28'></if>  The two are the same 
<if test='id != null and id lt 28'></if>  normal 
<if test='id != null and id < 28'></if>  Report errors 
<if test='id != null and id lte 28'></if>  normal 
<if test='id != null and id <= 28'></if>  Report errors 

對應關系：

gt           Corresponding              >
gte          Corresponding               >=
lt            Corresponding              <( Will report a mistake    The associated  "test"  Property values cannot contain  '<'  character )
lte           Corresponding              <=( Will report a mistake    The associated  "test"  Property values cannot contain  '<='  character )

 2.字符串類型

(1)for example ：

If you do not need to filter empty strings   Just judge null that will do 
<if test="username != null"></if>

(2)for example ：

If you need to filter empty strings , Add an empty string to judge    I won't support it  && and  || , So here we use  and  or  To make logical and or judgments  
<if test="username != null and '' != username"></if>  perhaps  <if test="username != null and '' neq username"></if>

(3)for example ：

If you judge whether the string starts with a special character , Ending, etc . Call directly String The corresponding method can be used 
    <if test="username != null and username.indexOf('ji') == 0"> </if> <!--  Whether it starts with something  -->
    <if test="username != null and username.indexOf('ji') >= 0"> </if> <!--  Whether it contains a character  -->
    <if test="username != null and username.lastIndexOf('ji') > 0"></if>  <!--  Does it end with something  -->

(4)for example ：

Whether it is a specific string , Some businesses need this .
<if test="username != null and 'hello' == username"></if>  perhaps <if test="username != null and 'hello' eq username"></if>
 Be careful ：
     <if test="username != null and 'hello' == username"></if> There is no problem with this form of writing when the parameter type is a string ,
 However, when the parameter type is non string type, it needs to be written as  <if test="username != null and 'hello'.toString() == username.toString()"></if>
 Just write <if test="username != null and 'hello'.toString() == username"></if> There will also be a great possibility of hanging up .
 Maybe you will say why non string should be written like this . This depends on the special needs .

對應關系：

eq                   Corresponding                 ==
neq                Corresponding                  !=
 3  Judge list Is it empty 
if Condition judgment can directly call the method of the object itself for logical judgment , therefore list Sentenced to empty . You can call .size()>0 perhaps .isEmpty()
 for example ：<if test="userList != null and userList.isNotEmpty()"></if> , <if test="userList != null and userList.size()>0"></if>
4 map Parameters are the same    If the value is taken  map.key(map Medium key name ) that will do

where label

標簽會自動判斷，如果沒有條件成立，那么在sql中就不會有...語句中 where關鍵字

如果有任何條件成立，會自動去掉多余的或者or。(我們不需要添加 1=1 這樣的侵入代碼)

usage ：

<select id="listProduct" resultType="Product">
    select * from product_
    <where>
        <if test="name!=null">
            and name like concat('%',#{
    name},'%')
        </if>        
        <if test="price!=null">
            and price > #{
    price}
        </if>
    </where>     
</select>

set label

和 where Labels 類似，在 update 語句中也會出現多個字段相關的問題。在這種情況下，您可以使用 set label 。它的作用和 where 標簽相似，只有在有數據時才設置。set 元素可用于動態包含需要更新的列，忽略其他不更新的列，set 元素在行首動態插入 SET 關鍵字，并刪除多余的逗號。

usage ：

<update id="updateProduct" parameterType="Product" >
    update product_
    <set>
        <if test="name != null">name=#{
    name},</if>
        <if test="price != null">price=#{
    price}</if> 
    </set>
     where id=#{
    id}   
</update>

trim 標簽

trim 有四個參數，即：

prefix： Prefix(以什么開頭)、

prefixoverride： 去掉第一個(如“and”還是“or”)

suffix： suffix(以什么結尾)

suffixoverride：去掉最后一個標記的字符(如“,”)

usage ：

<select id="listProduct" resultType="Product">
    select *from product_
    <trim prefix="WHERE" prefixOverrides="AND |OR ">
        <if test="name!=null">
            and name like concat('%',#{
    name},'%')
        </if>        
        <if test="price!=null and price!=0">
            and price > #{
    price}
        </if>
    </trim>      
</select>
trim  Used to customize the desired functions , such as where The label can be used trim  To replace 
<trim prefix="WHERE" prefixOverrides="AND |OR ">
  ... 
</trim>  
<update id="updateProduct" parameterType="Product" >
    update product_
    <trim prefix="SET" suffixOverrides=",">
        <if test="name != null">name=#{
    name},</if>
        <if test="price != null">price=#{
    price},</if>   
    </trim>
     where id=#{
    id}   
</update>
set The label can be used trim To replace  , function set The code in the tag , The effect is the same .
<trim prefix="SET" suffixOverrides=",">
  ...
</trim>

choose when else label

有時候我們不想套用所有的條件，我只想選擇幾個選項中的一個。MyBatis 提供了選擇元素，按順序判斷 when 中的條件是否為真，如果一個成立，則為選擇結束。當您在何時選擇所有條件時，如果您對所有條件不滿意，則執行其他媒體SQL。類似于 Java 的 switch 語句，按 switch 選擇，when 按大小寫，否則為 default.if 是與 (and) 之間的關系，并且選擇 Yes 或 (or) 之間的關系.

<select id="getUserList" resultType="com.it.bean.User" parameterType="com.it.bean.User">  
    SELECT <include refid="resultParam"></include> FROM User u   
    <where>  
        <choose>  
            <when test="username !=null and username != ''">  
                u.username LIKE CONCAT(CONCAT('%', #{
    username}),'%')  
            </when >  
            <when test="sex != null">  
                AND u.sex = #{
    sex}  
            </when >  
            <when test="birthday != null ">  
                AND u.birthday = #{
    birthday}  
            </when >  
            <otherwise> 
 	            AND u.age = #{
    age}
            </otherwise>  
        </choose>  
    </where>    
</select>

foreach 標簽

foreach 標簽通常用于在這種語法中。

collection ：collection 一個屬性有三個取值 list、array、map 三個，對應的參數類型有：List、 Array 、map aggregate ，我上面傳的參數是array ，所以取值為array

item ：表示迭代過程中每個元素的別名

index ：表示每次迭代在迭代過程中的位置(下標)

open ： 前綴

關閉：后綴

separator ： 分隔符，表示迭代過程中每個元素是如何分隔的.

<select id="listProduct" resultType="Product">
      SELECT * FROM product_
        WHERE ID in
            <foreach item="item" index="index" collection="list"
                open="(" separator="," close=")">
                #{
    item}
            </foreach>
</select>

bind label

bind在label中，value對應傳入實體類的一個字段，name屬性是賦予對應字段的變量名。在 value 屬性中可以使用字符串拼接等特殊處理。

usage ：

    <select id="listProduct" resultType="Product">
        <bind name="likename" value="'%' + name + '%'" />
        select * from   product_  where name like #{
    likename}
    </select>

sql Fragment tags

通過這個標簽，我們可以定義可復用的 sql 語句片段，在執行的 sql 語句標簽中可以直接引用。

這樣可以提高編碼效率，也可以有效地簡化代碼，提高可讀性。sql label 封裝SQL語言，包括要調用的Tag。

usage ：

<!-- Definition sql fragment -->
<sql id="orderAndItem">    o.order_id,o.cid,o.address,o.create_date,o.orderitem_id,i.orderitem_id,i.product_id,i.count
</sql>
<select id="findOrderAndItemsByOid" parameterType="java.lang.String" resultMap="BaseResultMap">
    select
<!-- quote sql fragment -->
    <include refid="orderAndItem" />
    from ordertable o
    join orderitem i on o.orderitem_id = i.orderitem_id
    where o.order_id = #{
    orderId}
</select>