要生成的sql：
作用是利用postgreGIS数据库函数计算两点（经纬度已知）的距离。

SELECT ST_Distance(
ST_Transform(ST_GeomFromText('POINT(-87.734087560562 43.770129071141)',4326),26986),
ST_Transform(ST_GeomFromText('POINT(-87.747382933006 43.759234252055)', 4326),26986)
);

最后成功的写法：

    <select id="selectDistance" resultType="java.lang.String" parameterType="java.util.Map">SELECT ST_Distance(<if test="lot_lat != null" >ST_Transform(ST_GeomFromText(<![CDATA['POINT(${lot_lat})']]> ,4326),26986),</if><if test="parkingPointStr != null">ST_Transform(ST_GeomFromText(<![CDATA['${parkingPointStr}']]>, 4326),26986))</if></select>

${lot_lat}是字符串：-87.734087560562 43.770129071141
${parkingPointStr}是字符串POINT(-87.747382933006 43.759234252055)

注意1：单引号的转义

方式a：如上使用<![CDATA[……]]>
方式b：使用xml的转义字符：

    <select id="selectDistance" resultType="java.lang.String" parameterType="java.util.Map">SELECT ST_Distance(<if test="lot_lat != null" >ST_Transform(ST_GeomFromText(&apos; POINT(${lot_lat}) &apos;,4326),26986),</if><if test="parkingPointStr != null">ST_Transform(ST_GeomFromText(&apos;${parkingPointStr} &apos;, 4326),26986))</if></select>

方式c:concat拼接的方式。
刚开始想拼接一对单引号出来，不行：

   <select id="selectDistance" resultType="java.lang.String" parameterType="java.util.Map">SELECT ST_Distance(<if test="lot_lat != null" >ST_Transform(ST_GeomFromText(concat(E'\'','POINT(${lot_lat})',E'\'','),4326),26986),</if><if test="parkingPointStr != null">ST_Transform(ST_GeomFromText(concat(E'\'','${parkingPointStr}',E'\'','), 4326),26986))</if></select>

后来坚持不懈，发现是这么写：

    <select id="selectDistance" resultType="java.lang.String" parameterType="java.util.Map">SELECT ST_Distance(<if test="lot_lat != null" >ST_Transform(ST_GeomFromText(concat('POINT(${lot_lat})'),4326),26986),</if><if test="parkingPointStr != null">ST_Transform(ST_GeomFromText(concat('${parkingPointStr}'), 4326),26986))</if></select>

这样也行：

    <select id="selectDistance" resultType="java.lang.String" parameterType="java.util.Map">SELECT ST_Distance(<if test="lot_lat != null" >ST_Transform(ST_GeomFromText(concat('POINT(',#{lot_lat},')'),4326),26986),</if><if test="parkingPointStr != null">ST_Transform(ST_GeomFromText(concat('${parkingPointStr}'), 4326),26986))</if></select>

这样也行：

    <select id="selectDistance" resultType="java.lang.String" parameterType="java.util.Map">SELECT ST_Distance(<if test="lot_lat != null" >ST_Transform(ST_GeomFromText(concat('POINT(',#{lot_lat},')'),4326),26986),</if><if test="parkingPointStr != null">ST_Transform(ST_GeomFromText(concat(#{parkingPointStr}), 4326),26986))</if></select>

concat函数解析后会自带单引号？我之前为了加上单引号还这么写呢：SELECT concat('''','POINT(116.289573 39.892352)','''')
SELECT concat('''','POINT(116.289573 39.892352)','''')
SELECT concat('POINT(-87.747382933006 43.759234252055)')
SELECT '''' || 'POINT(-87.747382933006 43.759234252055)' || ''''

这样不行：

    <select id="selectDistance" resultType="java.lang.String" parameterType="java.util.Map">SELECT ST_Distance(<if test="lot_lat != null" >ST_Transform(ST_GeomFromText('\''POINT(${lot_lat}) '\'',4326),26986),</if><if test="parkingPointStr != null">ST_Transform(ST_GeomFromText('\''${parkingPointStr} '\'', 4326),26986))</if></select>

sqlSELECT concat('\'','ddd','\'') from t_parking也有语法错误，应该写为SELECT concat(E'\'','ddd',E'\'') from t_parking，postgresql转义放在E'\要转义的内容'里。详见

注意2：单引号为什么不能用双引号取代

sql里单引号不能被双引号取代着用。

注意3：${} 与 #{}的区别

前者是直接把字面值加在sql里，完全是字符串替换，当作占位符。常常用来传递表名，字段名，如order by ${param}
mybatis在处理#{}时，会将sql中的#{}替换为?号，调用PreparedStatement的set方法来赋值，有预编译的机制。另外，传入的数据都当成一个字符串，会对自动传入的数据加一个双引号。
后注入的参数将不会再进行SQL编译。我们知道，SQL注入是发生在编译的过程中，因为恶意注入了某些特殊字符，最后被编译成了恶意的执行操作。而预编译机制则可以很好的防止SQL注入。

注意4：mybatis里写的一条sql里可以${} 与 #{}混着用吗？

比如这条语句：

SELECT ST_Distance(ST_Transform(ST_GeomFromText('POINT(117.343454 40.8978999)',4326),26986),ST_Transform(ST_GeomFromText('POINT(116.289573 39.892352)', 4326),26986))
FROM  t_parking
WHERE id='0652b36f115c42578d13ffa45c4589c8'

写成：

    <select id="selectDistance" resultType="java.lang.String" parameterType="java.util.Map">SELECT ST_Distance(<if test="lot_lat != null" >ST_Transform(ST_GeomFromText(<![CDATA['POINT(${lot_lat})']]> ,4326),26986),</if><if test="parkingPointStr != null">ST_Transform(ST_GeomFromText(<![CDATA['${parkingPointStr}']]>, 4326),26986))</if></select>FROM t_parking<where><if test="id != null and id != ''">and id = #{id,jdbcType=VARCHAR}</if></where>

可以吗？
当然是可以的，但${}里不能带,jdbcType=VARCHAR
写这样and id = ${id,jdbcType=VARCHAR}报错：org.postgresql.util.PSQLException: 错误: 语法错误 在输入的末尾\n 位置。
写这样and id = '${id,jdbcType=VARCHAR}'报错：内部错误。
写这样and id = '${id}'当然是正确的。
写这样and id = ${id}当然是有语法错误的。

另附postgis相关资料：
https://blog.csdn.net/qq_36588972/article/details/78902195
https://blog.csdn.net/zcc0618/article/details/72972188
https://blog.csdn.net/qq_36017609/article/details/85730172
https://www.cnblogs.com/kaituorensheng/p/4647901.html

CREATE EXTENSION postgis;
CREATE EXTENSION postgis_topology;

SELECT AddGeometryColumn ('t_parking', 'geom', 4326, 'POINT', 2)

update t_parking geom set geom = ST_GeomFromText('POINT(116.289573 39.892352)', 4326) where id='0652b36f115c42578d13ffa45c4589c8'
SELECT ST_ASTEXT(geom) geom FROM t_parking where id='0652b36f115c42578d13ffa45c4589c8'

select a1.attname as column_name,t.typname as data_type,d.description as column_comment
from (select a.attname,a.attrelid,a.atttypid,a.attnum from pg_attribute a,pg_class c where c.relname = 't_parking' and a.attnum>0 and a.attrelid=c.oid)a1 left join pg_type t on a1.atttypid=t.oid left join pg_description d on d.objoid=a1.attrelid and d.objsubid=a1.attnum;