一、SQL语句常用小场景

1.查询某个表信息，表中某些字段为数据字典需要进行转义

SELECTt.ID,CASEWHEN t.DINING_TYPE = 1 THEN'早餐'WHEN t.DINING_TYPE = 2 THEN'午餐'WHEN t.DINING_TYPE = 3 THEN'晚餐'END AS diningTypeStr
from student t

2.联表查询语法

select si.* from student_info  si
left join classed_info ci on si.classed_id = ci.id 

3.每个xml中可以自定义一个查询语句头

  <sql id="query">select ID, ORDER_NO, WX_ORDER_NO, SCHOOL_ID, GRADE_ID, CLASSES_ID, STUDENT_ID, STUDENT_PHONE,DINING_TYPE, FOOD_TYPE, PAY_TYPE, "STATUS", CREATE_ORDER_DATE_TIME, PAY_DATE_TIME,IS_DELETE, CREATOR, CREATE_TIME, MODIFIER, MODIFY_TIME from student_info</sql>

4.自定义查询语句

<sql id="query"><if test="gradeId != null and gradeId != ''">and t.grade_id = #{gradeId, jdbcType=VARCHAR}</if><if test="classesId != null and classesId != ''">and t.classes_id = #{classesId, jdbcType=VARCHAR}</if></sql>

5.批量新增

<insert id="createList" parameterType="java.util.List"><foreach collection="list" item="item" index="index" open="begin" separator=";" close=";end;">INSERT INTO t_student_info (id, name)VALUES(#{item.id},#{item.name})</foreach></insert>

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