SQL语句常用语法（开发场景中）

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

SELECT
        t.ID,
        CASE
        WHEN 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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