if:條件判斷語句,單條件
choose(when,otherwise) 相當於case when
trim(set,where):輔助語句,主要用來處理一些sql拼裝
foreach:循環語句
if條件語句
<select id=”findActiveBlogWithTitleLike”parameterType=”Blog” resultType=”Blog”>
SELECT * FROM BLOG WHERE state = ‘ACTIVE’
<if test=”title != null”>
AND title like #{title}
</if>
</select>
如果滿足if條件,則會拼接對應的sql
<select id=”findActiveBlogLike”
parameterType=”Blog” resultType=”Blog”>
SELECT * FROM BLOG WHERE state = ‘ACTIVE’
<choose>
<when test=”title != null”>
AND title like #{title}
</when>
<when test=”author != null and author.name != null”>
AND title like #{author.name}
</when>
<otherwise>
AND featured = 1
</otherwise>
</choose>
</select>
where條件
<select id=”findActiveBlogLike” parameterType=”Blog” resultType=”Blog”>
SELECT * FROM BLOG
<where>
<if test=”state != null”>
state = #{state}
</if>
<if test=”title != null”>
AND title like #{title}
</if>
<if test=”author != null and author.name != null”>
AND title like #{author.name}
</if>
</where>
</select>
這兩者效果一樣。
在使用if條件的時候,如果使用where條件,如果前面沒有條件,我們可能需要使用where 1=1
然後在if中使用and xxx
這種格式,但是使用<where>
條件的時候不用,mybatis默認會給<where>
條件增加where的sql,同時條件的第一個and語句會自動去掉。這樣就不用被動的寫 where 1=1
這種了
trim主要是用來去除某些SQL語法的時候使用,比如上面的<where>
條件
<select id=”findActiveBlogLike” parameterType=”Blog” resultType=”Blog”>
SELECT * FROM BLOG
< trim prefix= "WHERE" prefixOverrides= "AND |OR " >
<if test=”state != null”>
state = #{state}
</if>
<if test=”title != null”>
AND title like #{title}
</if>
<if test=”author != null and author.name != null”>
AND title like #{author.name}
</if>
</ trim >
</select>
如果當state爲空的時候,正常情況下sql爲:
SELECT * FROM BLOG where AND title like #{title} AND title like #{author.name}
但是trim會把where前面的AND或者OR關鍵字去除,變爲:
SELECT * FROM BLOG where title like #{title} AND title like #{author.name}
update user
<trim prefix="set" suffixoverride="," suffix=" where id = #{id} ">
<if test="name != null and name.length()>0"> name=#{name} , </if>
<if test="gender != null and gender.length()>0"> gender=#{gender} , </if>
</trim>
如說name和gender的值都不爲null的話打印的SQL爲:update user set name=‘xx’ , gender=‘xx’ where id=‘x’
在where前面不存在逗號,而且自動加了一個set前綴和where後綴,上面三個屬性的意義如下,其中prefix意義如上:
suffixoverride:去掉最後一個逗號(也可以是其他的標記,就像是上面前綴中的and一樣)
suffix:後綴
set元素主要是用來解決當滿足一個條件時,需要用where語句,而不滿足時,不需要
在update中類似的,用set元素:
<update id="updateAuthorIfNecessary" parameterType="domain.blog.Author">
update Author
<set>
<if test="username != null">username=#{username},</if>
<if test="password != null">password=#{password},</if>
<if test="email != null">email=#{email},</if>
<if test="bio != null">bio=#{bio}</if>
</set>
where id=#{id}
</update>
foreach元素:
<select id="selectPostIn" resultType="domain.blog.Post">
SELECT * FROM POST P WHERE ID in
<foreach " item="item" " index="index" collection="list" open="(" " separator="," close=")">
#{item}
</foreach>
</select>
open和close表示用什麼符號把元素包裝起來,separator是各個元素的間隔符
item是循環中當前的元素
bind元素
bind元素聽過OGNL表達式定義一個上下文的變量:
<select id=”findActiveBlogWithTitleLike”parameterType=”Blog” resultType=”Blog”>
<bind name="pattern_title" value="'%'+title+'%'" />
SELECT * FROM BLOG WHERE state = ‘ACTIVE’
and title like #{pattern_title}
</select>