What does mysql---where 1=1 mean

When the new colleague came, he asked me where 1=1 What's interesting , It doesn't make sense , I smiled. . Today, let's explain .

where 1=1

Let's start with a piece of code

<select id="queryBookInfo" parameterType="com.ths.platform.entity.BookInfo" resultType="java.lang.Integer">
 select count(id) from t_book t where 1=1
<if test="title !=null and title !='' ">
 AND title = #{title} 
</if> 
<if test="author !=null and author !='' ">
 AND author = #{author}
</if> 
</select>

The above code is familiar , This is the total number of qualified queries . stay mybatis Used in if Tag judgment where Condition after clause , In order to prevent the first field from being empty, resulting in sql Report errors . you 're right , When multiple query conditions are encountered , Use where 1=1 It can easily solve the problem that our condition is empty , So that's how it's written Is there anything wrong with that? ？

Many people on the Internet say , This can cause performance problems , May invalidate the index , So let's measure it today , Will you not go

Actually measured

title The field has been indexed , We go through EXPLAIN look down

EXPLAIN SELECT * FROM t_book WHERE title = ' And on earth ';

EXPLAIN SELECT * FROM t_book WHERE 1=1 AND title = ' And on earth ';

Comparing the above two, we will find You can see possible_keys（ Possible indexes ） and key（ Actual index used ） All use the index for Retrieval .

Conclusion

where 1=1 I'll go with the index , Does not affect query efficiency , We wrote sql Orders will be mysql Analyze and optimize into your own processing instructions , In the process 1 = 1 Such meaningless conditions will be optimized . Use explain EXTENDED sql Proofread , Find out where1=1 Such conditions will be mysql Optimized by the optimizer .

So we're in mybatis You can change the writing , Because after all mysql The optimizer also takes time , Although it's gone , But when the amount of data is large , It will still have an impact , So we suggest that the code be written like this ：

<select id="queryBookInfo" parameterType="com.ths.platform.entity.BookInfo" resultType="java.lang.Integer">
 select count(*) from t_book t
<where>
  <if test="title !=null and title !='' ">
   title = #{title} 
  </if>
  <if test="author !=null and author !='' "> 
   AND author = #{author}
  </if>
</where> 
</select>

We use it where Label substitution .

author ： Su Shi

link ：juejin.cn/post/7030076565673213989